You can paint over different colored 2k primers but you will need to be sure you have full coverage with your color coat.
I usually shoot 3 coats of color even if it looks like 2 coats got it. Some colors could take 5 coats or more before they will reach full coverage (where you won't see any variations or different shades caused by the underlying primer colors).
Most guys prefer to have a single color to paint over so there is less of a chance of any shading or color variation when spraying base. Another advantage is if/when the car gets a stone chip the underlying primer colors aren't glaring at you.
I like using reduced epoxy for a sealer coat for a few reasons:
1. It gives me a chance to see how the color coat is going to look.
2. Provides a consistent color to paint over
3. Provides better adhesion IMO
4. Provides better chip resistance
5. Can be wet sanded with 600 if necessary prior to base
Of course sealer coats are optional and this is just my personal preference.
General Discussion. Make yourself at home...read, ask and answer!
Joined: Fri Oct 20, 2006 12:40 pm
1968 Coronet R/T
JohnnyK, I get the impression that you are trying to combine block sanding serious surface irregularities with finishing sanding.
The two are mutually exclusive and should never be combined. Block sanding surface irregularities is a separate and distinct operation by itself. That is part of why I mentioned earlier in this thread about using too fine grit of a sandpaper for block sanding. If you use a finer grit of sandpaper for block sanding you are no making the paint flat, it simply follows the contours of the uneven paint. You need a more coarse sandpaper (nothing finer than 220 grit) for guidecoat block sanding and 160 to 180 grit has worked best for me over the years. What I have found that works best for me is first get the filler work close enough to being flat that it won't take a lot of primer or spot putty when it comes to block sanding. When the body work is done then spray on a couple coats of epoxy primer over the bare metal and sanded bondo. Give that epoxy a half hour or so to set up then spray on a couple/three coats of 2K high build. Next mist on some black guidecoat (or spread on some 3M guidecoat powder, good product by the way). You can spray on the black guidecoat over the gray primer while the primer is still wet or you can wait till later, really doesn't matter when. Let that 2K high build primer dry overnight and its ready to start sanding the next morning. At this point your only focus is on making the panels flat (no wavy areas/surface irregularities). Final sanding is a long way away and of no importance at this stage of the work. 160 or 180 grit sandpaper on the long board hand sander is in most cases the sander of choice for guidecoat block sanding primer. Keep sanding until all the guidecoat is gone or you hit bare metal or epoxy primer surrounding a low spot (with guidecoat still covering it). Stop immediately when you hit bare metal or epoxy primer because any more sandingin that area after that point is only working against you. Then spray on a couple more coats of 2K high fill primer and guidcoat block sand again
until all the guidecoat is gone or you hit metal again. It is very important to use guidecoat because it will visually expose low spots that you didn't know were there. Keep block sanding with the guidecoat until the whole car is gray primer with all the guidecoat is gone.
At that point the block sanding phase is done and finish sanding will be the next phase. So at that point you have a car that is all gray primer with 160 or 180 grit sand scratches in it. Mix up some more 2K high build primer only this time add a little reducer (No more than 10% by volume) to the primer and spray on a couple medium coats on the whole car. Getting runs are a little more serious at this point so try not to get any runs this time around. Let that primer sit at least a day to dry/shrink. At that point I go right to 400 grit (or even 600 grit if I'm not using a thinned down epoxy sealer coat just before spraying on the color coat). I do all the final sanding by hand with the 400 or 600 grit. You can wet sand or dry sand, it really doesn't matter which you choose. On flat panels I use a short hand block with the 400 or 600 grit (I like the large 3M black rubber squeegies for final sanding). On round areas I go with just the sandpaper alone (no rubber squeegy). Keep in mind here that before you even start the final sanding all the previous sandmarks are gone, covered by the last coat of primer and all the panels should be as flat as they're going to get. This final sanding's purpose is to remove any dust nibs and any orange peel in the last coat of primer. You should be able to scuff the whole car in less than a couple hours. (again keeping in mind that all the hard work was already done when you get to the final sanding part).
I also agree with everything 68Coronet told you. On a full paint I in most cases will spray on a medium coat of lightly reduced epoxy primer as a sealer coat. I consider that sealer coat good insurance. It should be sprayed on just before you start shooting your color coats. (give the epoxy sealer coat 20 minutes to a half hour to dry/flash then start shooting your color coat). There should be NO need to sand the sealer coat).
I hope this makes sense and helps you.
Oh, and that primer screw up on the quarter panel scoop is not from contamination. You spent a second too long in that one spot and the primer "puddled". Sand it flat and you're good to go.
Thanks tons guys! Phil, thanks for the long write up! You wrote this
"At that point the block sanding phase is done and finish sanding will be the next phase. So at that point you have a car that is all gray primer with 160 or 180 grit sand scratches in it. Mix up some more 2K high build primer only this time add a little reducer (No more than 10% by volume) to the primer and spray on a couple medium coats on the whole car. Getting runs are a little more serious at this point so try not to get any runs this time around."
THIS is where I am at, and this is where I ran it.. The car was already guide coated and flat with 180, so I shot some reduced 2K on it.. Unfortunately, I ran it, and there were also those few places I mentioned that had deep scratches on it.
So, could i potentially sand those runs flat with 180, and switch to 320 and 600, or is spot priming the 180 grit sanding scratches with more 2k necessary?
edit: I meant to add, I'm a little worried about adding a sealer, just in case of runs!
OK, now we're " on the same page. " The run on that inside angle (where the bumper goes) can be sanded by hand (no sanding blocks). I would probably use 80 grit by hand to get most of the run (s) out in that spot then finish if off with 180 then maybe 220 grit. That will require at least one more coat of fill primer. The run by the quarter scoop - if there isn't filler behind it then you'll probably hit bare metal sanding that run out so that will definitely need more primer in that spot. The run where it appears to have "puddled" some you sand with 180 on the longboard sander and it will need at least one more coat of primer.
Thanks man! That first one on the angle, if we're talking about the same thing, that's at the top/back of the window, right where the drip rail molding curves around and heads straight down.
So you just mean spot prime, right? Then I can level it down flat with the rest of the car and finish sand? (I hope!). Should I level the spot priming with 180, then continue on finish sanding? I hope that's what you mean! Sorry for the basic questions. Never been this close before! haha
no they are not contamination that happens when the gun gets too close or you were spraying with the pattern too narrow
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