Block Sanding Primer Do’s and Don’ts

Block Sanding Primer Do’s and Don’ts

-Hey, this is Donnie Smith. And welcome to this lesson
on block sanding primer. In this lesson, we’re
going to give you some of the do’s and
some of the don’t’s when it comes to block
sanding your primer. Now, I’m sure the
goal that any of us have when it comes to the end
result, the finished product, once it’s been painted, is for
it to have that flawless look, that show-quality
paint job look. A lot of it comes
from your body work. How well did you block
your body work out, and how smooth and
level did you get it? But block sanding
your primer also plays a big role in how
straight of a finished product you’re going to have. Now, in the last
video in this series, we left off– we had
sprayed some epoxy primer. And then we applied
some primer surfacer. And now, it’s ready to block. Now, let me give
you a little tip. We’ve already got the
car all masked off. So let’s leave it masked. Now, I know in some
production shops or in your certain
situation, you may not always be able
to leave the car where it’s at until
you’re done priming. But if possible,
leave it masked off. Even leave the plastic
over it, if possible. Remove just what plastic
you can to move the car. Maybe take the plastic
off and move it out of the spray booth
or the prep station, because the goal here
is to do everything with the least amount of effort. We want to do things
right, but why work harder if
you don’t have to? Now, at this point, we
may have a good idea– especially if
you’re experienced, you may have a good idea that
this is going to take one block and you’re done. And that may be a
different story. But let’s assume
that we’re not sure. We’re going to
block it out to see if we’re going to have
to prime and block again. If you should have to prime
again, it’s ready to go. You clean it up and you
can spray more primer. So leaving it masked off
during your primer stages is going to save you time. Now, when you go to
paint, obviously, you’re going to have to
take all that off, wash it, clean it so you
don’t get a lot of dirt in your paint job. But for now, leave
it masked off. And the second tip I want to
give you is using guide coat. Don’t be afraid
to use guide coat or think that you’re too
good to use guide coat. I still use guide
coat, and I know there’s a lot of people
that have been doing this for years and years that use it. However, I know there’s
some people out there that don’t think that
they need it anymore. And maybe some don’t. But I would recommend
going ahead and using it. It’s a tool to help you
to make your job easier. Remember, to do this
with the least effort and using what tools
you have available is going to make
your job much easier. Now, what guide coat is–
that’s a spray or dry powder form that goes on the surface. And it’s a contrasting color. Usually, your primers are
light, so your guide coat’s going to be dark, usually black. And you put that on there. So if there’s any pin holes,
scratches, or minor lows, that guide code is going
to stay in that area, and that way, you know
there’s a problem there. There’s a small low. You see a line of
guide coat, you know that’s a
scratch that you’re going to have to
do something with. I recommend using guide
coat during the priming and even some of the
body filler stages. Now, let me give you a tip
of not to do– a don’t. And I’ve seen this a lot. You see an area that has
guide code and in your mind, you think the objective is to
remove all the guide coat, sand all the guide coat
off, which it is. But if there’s some guide coat
left and it’s not sanding out, don’t sit there and
dig the block into it. I’ve seen that time
after time, where you’re sanding on a fender
or something like that and you tip the
block on the edge just to get that guide coat out. Well, that defeats
the whole purpose. You might as well
not even guide coat, because it’s not
doing you any good. Now, when given
the block level, it may sand out before you
sand through the primer. Or it may require that you
have to prime and block again. Now, guide coat
comes in two forms. They have the spray
that you can use. It comes in an aerosol can. And they also have a dry form. And if I’m doing a hell
damaged job or a real big area, I’ve even mixed
some in a paint gun and sprayed the entire area. But there is a trick
if you’re going to mix it up in the
spray gun to spray. You don’t really want to use
any type of enamel or base coat or anything like that,
because it stays tacky. And it’s just going to
clog up your sandpaper. If you’re using a spray
gun on a large area, I recommend some
type of– if you can find– old lacquer primer. That works really
good, because it dries and it doesn’t really gum up
your sandpaper like paints do. I know they make
black lacquer primers. That would work good. I would over thin it and
guide coat the whole area. You just want to
mist it on there. You don’t want it on too heavy. Now, with that said, I
know there’s some of us out there that
tries to save money. The guide coat’s a
little bit expensive, so we go to Walmart and
get that $0.99 spray can paint thinking
we’re saving money. And I used to do the same thing. I’m guilty of it myself. But if you’ll
notice, whenever you go to sanding that, it
just gums your paper up. you sand twice as long,
go through twice as much sandpaper. But when you have a
big area, really, you don’t want to spend all that
money on sandpaper and all at time blocking that area. So I really don’t recommend
using that cheap spray paint as a guide coat. It does the job. It definitely will
identify the lows. It works like a guide coat. But it sure does gum
up your paper, cost you more sandpaper and a lot
of extra effort and time. Now, let’s talk about what
grit of sandpaper you use. Now, this is going
to kind of depend– are you going to have to
prime it again or not? Sometimes, you may not know. If you do it often,
you kind of know. My body work is kind
of a little bit rough. I think I’m going to have to
prime and block it two times. And sometimes, it
may even take three. And with time, as you get
better, more experience, it gets to the point
where usually, you’re going to prime and
block it one time. If you know your
body work’s just going to need prime
and blocked one time, I would start out sanding
with 320 grit paper. You can do that dry or wet. But I do recommend blocking
your primer by hand. Now, you can use a DA, and there
are systems out there for that. And your production shops may
use DAs for blocking primer. But I’m still a little old
fashioned in that sense, and I like to hand block
the primer, especially if you’re a restoration person
out there wanting that show quality job. You’re definitely going to
have to block that out by hand. Now, if I’m not sure or
I think it’s probably going to take another
prime and block, I won’t spend my time
blocking it with 320. I’ll go ahead and
drop down to 220, which is a little bit coarser. It’s going to block it
a little bit faster. Then you can block it out. And once you’re done blocking,
you’re ready to prime again. And then you can
block again with 320. Now, what if we
block it out with 220 and it blocks out perfect? Do I have to prime it again? Not necessarily. If it blocks out
perfect with 220, go ahead and go over
the surface with 320 to remove those
220 grit scratches. And then to final sand it, you
need to go over that area again with 500 grit to remove
the 320 grit scratches. So if you’re a beginner,
this is what I’d recommend. I would block it out with 220. If it blocks out
fine no problems, you hadn’t sanded
through the primer anywhere, go ahead
and switch to 320. Then block it out with 320. And then final sand
it with the 500. That’s what I’d do. That’s somewhere to start. As you become more
familiar with this, you’ll learn your
body work and know if it’s going to need one or two
times of priming and blocking. If you know it’s just
going to need one time, you can eliminate or
skip the 220 grit stage. Now, let’s talk about selecting
the right block to block sand. We know what grit to use, but
what kind of block do we use? We were at the lake the other
day, and it was a little windy. Jake, my son, he
was dying to ski. So he wanted to get out
there and ski one afternoon. So we went out there, but
it was a little bit rough. As my boat hit
those waves– and it kind of bounces
around a little bit. He was able to ski, but
it sure did wear him out. So when it comes to blocking,
think of a boat in water. The reason for this
story is to explain how a small boat is going
to follow the wave patterns. And that’s kind of like blocking
whenever you’re blocking. If you’ve got a low
area, your block’s going to dip down with it. It’ll sand the guide
put out because it’s following that shape. Now, if you’ve got a bigger
boat, you’ve got little waves, it’s not going to
affect it as bad. And just like with
blocking, if you’ve got a small low area
right here, that block’s not going to dip down in there. It’s going to go straight
over that low area. And that will help to
identify that low area and not just block
the guide coat out. So when blocking, use as
long block as possible. Now, this is a little
bit exaggerated, but use a longer block
whenever selecting the block. You may be thinking, I’ve
already done my body work. I did a good job on that. Why am I worried about
lows at this point? Well, there are still going
to be some imperfections, no matter how well
you do your body work. Even a feather
edge area– that’s one of those real gradual dips. You see the rings. You may not be able
to fill it too good, but if your block follow that
pattern where it goes down– you look at the side of the car. You have that finished
product, looks great. But you can look
down the side of it. You can just see that
little wave in there. And that’s really
something none of us want. We want that perfect,
smooth, straight look– that show quality job
that we’ve talked about. So to do that, use a long block. Now, talking about selecting
blocks in boats in the water, it wouldn’t make any
sense– obviously, we’re not going to
go get a cruise ship and put it in a lake
like we have here. It wouldn’t fit. So a long block is not
going to fit every area that you’re sanding. So you’re going to have
to select the right block. So if you’re setting
on curved areas, by body lines, small areas that
you can’t get a big block to, obviously, it’s going to
take different blocks. So it’s good to have an
assortment of blocks that fit the shape that
you’re working on. But with that mind,
whatever area it is, use the biggest block that
you can that will fit– fits the contour,
fits the shape, and fits the surface
that you’re blocking out. Now, the purpose of blocking is
to sand down all imperfections. So if you just try
hand sanding it, again, your hand is going
to follow all those waves and patterns. So try to eliminate
using your hand. There may be a few
areas you have to, or they make some
flexible blocks that will get those areas. But in general, use
a hard block that’s not going to follow any of the
patterns or contours that’s in the surface. Another question
I’ve been asked when blocking– how do I
know when to stop? Well, that’s a good question. But if you use your
guide coat, you know when to stop, because the
point is to sand all the guide coat off, without dipping
your block in it, of course. But if all the guide coat
sand’s off, you’re good to go. You’re good to switch to
320 if you’re using 220, or you’re ready to final
sand that area with 500. Now, if you still
have some high areas, if you sand through
your primer and you start hitting your body
filler or your metal, that is another indication
that you need to. stop. Once you get your
metal, it’s not going to sand down any further. You’re just going to start
sanding the surrounding areas lower, and that’s going
to cause more damage than good. So once you sand through
your primer, stop. So if I’m block
sanding a fender, continue blocking until all
the guide coat’s sanded off. Let’s say there’s a
little spot left there. Well, you can go ahead and
block that area until you either sand the guide coat
out, until it’s removed, or you sand through the primer. That’s two indications
when it’s time to stop and you’re going to have
to prime and block again. Now, let’s talk about
blocking techniques. You don’t want sit there with
real short strokes like that. It’s going to make
it look choppy. You’re going to see that
in the finished product. If possible, use longer
strokes– nice, smooth, longer strokes. And sand at 30 degree angles. And you can continue
your cross sanding like we talked about
in body filler. So sand it in one
direction about 30 degrees, and then come sand in the other. So cross sanding at
30 degree angle– that is what’s going to produce
that show quality level surface that we’re looking for. And another thing
when blocking– we’ve already talked about this,
about not tipping on the edge to sand your guide coat out. Keep the block
surface flat on there. Don’t take tip it on
its edges or on the ends to get guide coat out
or for any other reason. Keep that block straight and
level and flat on the surface when you’re sanding. OK. In this video series,
we got the fender. We have it epoxy
primed, primer surfacer. Now, we’ve blocked it out,
with 220 if necessary, and then come back with 320 to
remove the 220 grit scratches. And now, we’re going to sand
that area with 500 grit. And if you remember,
the rest of the fender is already sanded, because
we did that before we primed. We’ve got the area that’s going
to be painted sanded with 500, and then the area that’s
going to be blended and clear coated only with 800. So now, when we have that
sanded with 500 grit, that fender should
be ready to paint. Now, we can prep it and get it
ready to put in the paint shop. Well, I hope you found
this video useful. If you did, be sure
and go down below, give us a thumbs
up, give us a like. And subscribe to this channel if
you’re not already subscribed. If you have any
questions or comments, go down below in
the comments section and leave your comment there. Thanks for watching. And be a resource
for someone else. Share these videos
with someone else that may want to learn the
basics in auto body and paint. And remember, if something’s
worth doing, do your best and have a blast doing it. Hey, before you go
anywhere, be sure and check out some of my
other videos and playlists.

100 thoughts on “Block Sanding Primer Do’s and Don’ts

  1. I watch my son do this. Now I understand more from your video. I want to help restore a Bertone X 1/9 . Tomorrow, I sand the hood he primed Saturday…carefully. Thank you for explaining so much. We did a VW Bug last month. Beautiful. Learning auto body work is so fun, I barely cook anymore. We have nice cars but we are hungry.

  2. Starting with 320 grit ensures that the paper will roll into and over imperfections.  
    START WITH COURSE PAPER  80-150.  Course paper properly cuts the highs and levels to the lows.  Too fine a paper (even with a block) will often roll over the imperfection.

  3. So When i spray my epoxy primer, i let it dry than i proceed to spray primer surfacer than i let it also dry. So right from the beginning do i put on guide coat and sand with 220, or do i sand first than apply guide coat over what i just sanded?. I'm confused of the order in which i lay down guide coat  and sand.

  4. Hi so you sand your primer with 320 then 500.  When I sand with 320, the 320 removes the primer, so I would have to spray it with primer again to sand with 500?

  5. I just bought new panels for my 95 Ford F350.  The panels have black primer on them.  I am told I have to fully remove the cheap primer and apply primer again before painting.  Is this true or can I just sand with 320, splash some Johndeer primer on it then sand it with 500 and apply one or two coats of my single stage paint?

  6. Awsome thankyou im in australia im moulding monaro bonnet scoop in and found your video very helpful i dont have any skills in body or paint work and have no help the people i no with experience back yard painters seem to no alot about nothing you have sumed the guide coat up for me as i couldn't work out the purpose of sandind every bit of black paint off and still be left with low points with no paint thanks your presentation is clear helpful and detailed keep up the good work

  7. Can you prime over the low spots of guide coat? If you hit metal and you don't want to sand the low pots (which makes sense) what are you supposed to do? Either needs to be cleaned off or primed again over. I use 3m dry guide coat, I tried washing it with soup and water, tried rubbing alcohol, it didn't work.

  8. Amazing video. I worked under a professional for a short time years ago and your video reminded me of the things I forgot. Very comprehensive and well made. Thanks for taking the time. Much appreciated.

  9. So you can't show us as you're speaking how you do it? I'm trying to figure out if I need an epoxy under the primer & if you shoot primer over the 800 grit sanded spots. Can't seem to feather the paint enough not to see a line !

  10. What are your tips on putting glaze over primer? Steps starting from what sand paper to reprep for the glaze?

  11. Thank you for the good video. Restoring my 1971 Lemans Sport convertible, and just cannot afford to pay the 10-15 grand I've been quoted by several for a good street paint presentation. Most shops seem to think concours or nothing. I found a mobile soda blasting company that will do soda blast at my garage. The car is relatively clean of rust with some typical body dings and some wavy panels. I figure if I can do the bodywork and prep, I can save a bundle to then have it professionally painted. Again, thank you.

  12. I am painting my 91 Cadillac Brougham. I am using slick sand feather fill g2. When I block sand the panels. I keep sanding thru a few edges to metal. I have an epoxy primer under it. I have two coats of epoxy and two coats of feather fill. Do I need more coats of feather fill to avoid this? Maybe tape the edges? Thanks.

  13. thank you. I'm new when it comes to sanding and blocking. Great tutorial now I know what type of block to use in different places

  14. FINALLY, an instructional video that explains everything I was wanting to know about prepping, priming, blocking and using guide coat!!! Subscribed and thankyou for also considering the people on a budget! I already threw a lot of money out on the rustoleum paint roller technique only to go back to step one again because I either didn't prep right or because crutial information about prepping were left out. Lots of wasted sandpaper, paint and disappointment. I wont be using a spray gun but the techniques you described will help a lot

  15. Thank you so much for the tips. I'm restoring a 1959 Glastron Fiberglass boat and just primed the hull and wasn't exactly sure what to do next to prep for the finish coat. This video was exactly what I need to see.

  16. I"m a retired Auto Body technician, yes we are Auto Body Technician's not Body Men. Any way this man sure knows his trade. Thanks for the great video….Canada.

  17. Thank you. I have been learning a lot here lately in preparation for my first paint project. I have an extremely amateur question:

    Do blocks wear down easily and should i get more than 1

  18. I was taught to block primer with 320 grit. If everything is okay, then I would wet sand it by hand with 400 grit and that was the last thing to do as far as sanding goes.

  19. I wouldn't risk using an air-powered tool like a DA sander to block with. It's too easy to mess something up, like digging into the primer, and just overall ruining whatever you primed. I tried to "block" using a DA sander once, and yeah, I screwed my hood up!!!! So I had to basically do everything all over again. DO ALL BLOCKING BY HAND! Just my very honest opinion.

  20. So, I guess you can't block sand epoxy primer then? Is it only possible to block sand polyester primer surfacer or 2k urethane primer surfacer?

  21. Block sanding by hand is the way to go, I agree. Do you re apply guide coat in between 220 and 320 stages?

  22. great series by a real professional, keep up the good work, I have not only learned a lot but also refreshed my memory on a couple of basic methods

  23. Outstanding. To say the least… Im going to use his methods . like NOW… Yes ive tipped the block.. Yes i used tbe walmart paint. . ive done the dont's… It is now time for the do it correctly. Tnsbk you

  24. Tips was a lot of help was unsure what to do when coming to bare metal when block sanding it 16 restoring a 65 mustang 289 4 speed k code car

  25. This video is incomplete because you do not show how to check your work using a solvent cleaner. Which seems like it is the most important step and also the real answer to "When do I stop?". Also, the example of a feather edge at 8:16 is terrible. That is not a feather edge… that is an edge map waiting to happen. If you primer a feather edge in that condition, you will be applying a second round of primer to the repair area guaranteed. Here is some thing else for the MSO techs who are comission paid book time out there: What are you getting paid for block sand and prime??? I almost never got time on my sheet for block sand and prime. Which is funny because the insurance companies are pushing HARD for repairs over placement parts. So, as a refinisher, I am literally working for free every time I prime and block sand a repair panel. Or I am paying for a prepper that is block sanding and priming cars for free… still costing me (us) money either way. But there is more…. I am also missing the repair TIME for the block sand and prime operation. So lets look at EVERYTHING this missing TIME effects… 1. Missing time means I am missing pay on my pay check. (That's ok, I should enjoy working for free???) 2. Missing time means the vehicle has less time allocated to cycle time. 3. Missing time means, time missing on the sheet that pays for MATERIALS. I wonder if any of the refinishers reading this have ever heard the following from an owner or management… This car is late (cycle time issue) Customer is paying for a rental (cycle time issure) We need to have a meeting with the paint shop about material costs/usage (missing material costs that should be billed out to the insurance company. This is always funny to me because ME not getting paid is fine… but the SHOP missing money, unacceptable!) Not to mention… what if you have to reprime a car??? You just primed a car twice FOR FREE. And consumed DOUBLE the material… gee, I wonder why material costs are so high??? Oh yeah, and you got paid even less because block sand and prime was not a line on your sheet in the first place. So you primed and blocked a car twice for FREE. It is actually worst than free, because you missed out doing a repair procedure you actually get paid for. But hey… the insurance company saved money! Ever wonder why the body men make way more money than you do? (Cause you are fixing their terrible body work at YOUR own expense while they are working on their next car.) Don't you love how the body men work less hours than you and still make more money? That's ok… just keep working for free. And hey, can you come in early tomorrow? Or stay late? We need to get these cars done. And THE BEST LINE EVER said to me… Hey bro, you can "make it up in volume" … REALLY??? HOW??? Every car is missing time. You can't make it up! And further more, why am I "making it up" it shouldn't be missing in the first place! Body men should block sand and prime their own repairs… PERIOD. They should be held accountable for their repairs. Do body men match color for me??? No, that's my job. I wouldn't expect them to do that. So why is the paint shop expected to finish a body mans job for him? You techs should stop and think about how much time a week goes into block sand and prime. You are being scammed, badly! I know that not all shops are like this. I am sure some shops get time for block sand and prime. However, all the shops I have worked for over 20 years now, do not get block sand and prime time. What do you commission techs think about that? Do I have a point? Do any of you have a story you would like to share with us? Is this only happening to me?

  26. As I have had some previous diy experience with bodywork and spraypainting your tips an advice have worked for me,the guide coat bit definitely worked on my m/cycle fuel tank,and the end result was better than all my previous diy work,true,proper prep is required for good end result,kind regards.

  27. Question, what’s the best action to take if I didn’t block the primer enough, I put a base coat, now the primer is raised through the base coat. What’s the best grit of sand paper to fix this, and do I wet sand or dry sand??? This my first time doing this

  28. I've been an autobody hobbyist and have run a small shop since the mid '70s and it's good to see some of the tips you have here. It's always good to learn new things. Thanks for posting!

  29. Thanks very helpful and hope to be trying is soon. Also a delight to watch such a video without the bad language


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