I was watching an Eastwood video on block sanding and they talked a little about primers. They said that 2k epoxy primers would shrink if applied to thick. Is this true? And what happens ? Cracks in the primer?

I am using HOC KD 3000 series grey primer. I applied 3 coats and most of it will probably be sanded off when I block it. I wasn't going to sand a lot in the door frames, window frames and some of the other hard to sand areas. I was going to use the maroon scuff pads in those areas.
Will I have problems in those areas if the primer is to thick?

 

Yes they will shrink. A primer with added solvents(thinner) will shrink more than one without as the solvent escapes. If you sand the primer before it is cured, solvent will continue to escape causing sand scratches to become more prominent. They will show up in the topcoat.
3 coats is not too thick, unless you really hogged it on there. Let it dry till you can't smell it anymore, then sand normally.

 

I didnt add any solvents which i assume is refered to as "reducer". Only primer and hardener at a 4-1 ratio. I primed it sunday and started sanding yesterday evening.

 

First of all you shouldn't be using HOK KD3000 Surfacer/Sealer as a base primer...it is too thin. It is made to be a surfacer, light duty primer to fill scratches and cover filler and/or sealer which can be sprayed on just prior to basecoat. What you want is something like SEM Metalock or Advantage 325 high build 2K DTM primer on the panel prior to sanding/blocking. For an even better build try Clausen All-U-Need polyester primer. As a general comment on paints...if you apply too many coats, the coats on top will dry before the first coats fully "gas out". This causes all sorts of problems including crazing, cratering, and other non-good things.
To your concerns about shrinking...there will always be "some" shrinkage as the solvents gas out, the point is "when" does the shrinkage occur? If the shrinkage occurs prior to sanding then who cares...however, if the shrinkage occurs during/after sanding then you have issues. In the days before activators and catalysts, lacquer primers took a long time to dry and settle out especially if you rushed the job and shot it on too heavy. You could finish a paint job over a thick lacquer primer application and in a couple of days sitting in the sun the paint would look like a dry river bed. Not so much today with the primers commonly used.

 

Yes and wait a generous amount of time between coats which will also aid in the drying process

 

Ok good info thank you. I used the kd-3000 because I believe you recommended using it in other posts here, because it is good quality. And on HOK advertisement said it can be used as a high build, medium build, or just a sealer depending on amount of reducer used.

I had already primed it once with a Summit brand high build 2k primer, then blocked /sanded it. I had to redo some of my previous bondo work mostly because I had sanded to much out and had low spots. So I decided to use the kd-3000 because it is probably a better quality product than Summit. So anyway I will sand and block it again and repaint it with the kd-3000.

 

all of Summits paints are Sherwin Williams products so don't be afraid of quality

 

The best use of KD-3000 is as a sealer just prior to color coat. It will hold out any bleed through and it will create a solid color base surface on which you can apply your color coat, either base coat or catalyzed single stage. KD-3000 flows like crazy and leaves a super smooth surface which makes it a great base for your color coat. Sorry if you got confused with this and the high build primer.
You should be using your 2K high build primer until such time as you have the surface level, no high spots and no low spots. Block it down until you have removed all visible scratches...a guide coat will help here. Now is the time for the KD3000. Mix it to match your color coat and spray a couple of coats on. I use a final block at 600/800 grit. By this time your surface should be close to perfect. Prior to final paint, prep the surface with a final wipe down with pre-paint solvent, tack, and get ready to paint. Spray enough KD3000, thinned as a sealer, to give yourself an even color then after about a 30 minute flash time go ahead and spray your base coat/color. I think that you will really like the results.

 

Good to know, thank you.

 

Thanks again Charlie for taking your time to help a newbie to this bodywork and paint world.

 

Charlie, How do you remove and neutralize surface rust (some minor pitting)on panels before applying Clausen All You Need HB primer? I am thinking of using it. The instructions say don't use any metal conditioner.
Thank you.
Bob

 

There are many rust convertors available these days, I use a product from KBS called Rust Blaster that works well. I have recently bought a 5 gal bucket of another product called Evaporust which I have used with very satisfying results. The Evaporust is comparatively inexpensive, reusable, and environmentally safe. I would use the rust convertor to remove/neutralize the rust first and then clean/sand the metal prior to apply the Clausen or other primer. Most rust convertors leave a phosphate layer on the metal which would inhibit the bonding of the primer but this layer can be easily removed with a sanding/scuffing of the surface prior to primering. As for pitting, if you use the Clausen All-U-Need, most all pitting will be filled.