I painted my cab with a low VOC DTM primer. I am sanding and redoing some of my shabby body work, and when done I will repaint it with the same primer.
After that is done I want to spray a high build primer from the same supplier. (Summit racing). Can I just scuff down the DTM primer with a scuff pad and then spray the high build. Or should I sand it with say, 320 grit paper.

 

You really need to check the manufacturers recommendations. Most primers have a window of opportunity to recoat without sanding. After that window closes, sanding will be required to have proper adhesion. How aggressive that sanding needs to be done will depend on the manufacturer. Some may say a scuff pad is fine, but personally, I'd go with something more abrasive. I would not want to have to redo all this work because I was too lazy to sand thoroughly at a time when it would be easy to do.

 

What is the point of doing the bodywork and THEN spraying it with high-build primer? The purpose of high-build primer is to give you a heavy coat to fill the imperfections...it sounds like you have already reworked your "shabby body work" so you are past the point where high-build primer is warranted. Most 2K primers on the market today are formulated to be "high-build" or primer surfacer or sealer...it all depends on how much you thin the primer when you spray it. If the surface is free of defects then all that is needed is a block sanding with 400, wipe down, and a couple of coats of sealer (I use House of Kolor KD-3000 line) prior to spraying the color coat.

 

Maybe an ignorant and oft repeated question, but I have learned it to be the best route to not messing something up. Your photo has me itching to be at the point you are. I am repairing the cab floor. The rest of the cab is in pretty good shape, but is a mix of old primer over original paint. I have been sanding the PO's primer coat down, as it had set outside and was oxidized. Best I can tell is that I am sanding off the build primer, as it comes off real easy, then beneath that is a harder primer. Leads me to my main question. Should I try to get my surface uniform (meaning all bare metal) or will it be ok to coat primer over top of a mixture of primer, original paint and bare metal. I have no idea what kind of primer was used by the PO.

 

As paint oxidizes it becomes powdery. This process begins at the surface, where the paint is exposed to the air, and then progresses inward towards the metal. If you have sanded through the oxidized layer and found "harder" primer/paint then you have a stable base on which to build. Today's primers are usually 2K which means that they are catalyzed. A catalyzed primer will not interact with the paint under it, it holds out base paint/primer/filler so that there will be no bleedthrough. While many builders will remove every layer of paint/primer down to the metal, this is only necessary if the original paint is damaged or if the builder wants to see what lies under it all. I would use a good 2K primer such as SEM Metalock or Advantage 325...straight out of the can they are high-build, reduced they are either a primer surfacer or sealer. Very versatile and fairly easy to sand, wet or dry.

 

Been a long time since I have done body and paint work. But, since you are in the priming and blocking stage to get rid of any imperfections, have you tried applying a guide coat? It will help you find small imperfections, edges, pin holes, etc.

 

I haven't used the guide coat yet. The primer I used has a slight gloss to it and when blocked the low spots stay glossy.

This is the first time doing this so I have a lot to learn. From what info I have gathered the way to prep for final paint should be..
1-Sand old paint and applying/sanding body filler where needed.
2- apply epoxy primer to seal
3- apply high build primer to block sand
4- apply a sealer
5 - then final color.

Some of the areas where I used filler I sanded down to far so they are low. So I am adding a little more filler to those spots.
In the process of sanding this first coat of primer I have exposed bare metal , so I am going to spray it again with the epoxy primer. It is a high build epoxy primer so maybe I can use this to block it instead of buying a "high build" primer.

 

I've only painted about 6 cars when I was younger. Methods have changes some since then so don't take this as absolutely correct for today's materials. My father fortunately new a professional painter that would provide advise and help out a bit when we were in a jam.


Basically we used to:


1. Strip entire vehicle to bare metal then sand the bare metal with 80 grit paper. The bare metal would get cleaned with an etch. Then prime to seal the bare metal. On a few we didn't strip, but sanded the exiting paint with I think 320 grit (not sure, been a while)


2. We didn't put body filler (bondo) over any paint. Those areas were always bare metal (again) and sanded with a very rough sand paper, I don't recall if it was 36 or 80 grit. Now I understand, if you epoxy prime/seal over bare metal, the body filler can go directly over the epoxy primer.


3. After metal work and body filler was as best we could do it was several times of prime and guide coat. Block sand and repair small imperfections with glazing putty if needed in between. We weren't professionals so sometimes it seemed to never end.


4. After final block sand of primer it was sealer, color and clear all in the same day.


5. Finally the labor of color sanding, polishing, glazing, and eventually after about a month, a coat of carnuba wax.