This is a pretty specific question but wondering if anyone has experience or thoughts on PPG K36 vs. K38 primer surfacers. In reading other forums, it seems some people think that because K38 is a high build surfacer, it has a higher solids content and therefore less shrink back. Local shop stocks K36 but has to order K38 which is also more expensive.

Would one month sitting in the sun be enough time for either of these to shrink before blocking, seal, and finish coat?

I realize there are probably cheaper options that are just as good but I'm pretty limited on what is available at the local paint shop and want to keep all the products in the same paint family since I'm a novice at this.

TIA.

I would hope that ppg would let you go to paint after a month. My experience with k36 is simply that it isnt very strong. If you dont sand much of it away, the surface paint can be vulnerable to chipping. Iam a complete amateur though having painted only 3-4 cars. Hope some more experienced members will chime in.

I've had good luck with K36, especially over DP90LF, and within a day or two after the DP90 coats. My experience is that it goes on thick and splotchy, requiring more sanding than the old days with acrylic laquer primer surfacer. A bit of reducer helps a ton. So I take quite a bit off while sanding. Just sprayed the hoods, eyebrows, and engine lid yesterday.

Thanks! Yes, I'll be spraying it over DP50LF (the gray version of DPLF). I also plan to let the DPLF sit at least a day before shooting K36. How long do you let the K36 sit before block sanding?

Can't remember if it was Kevin Tetz or someone else on youtube who talked generally about surfacers not being very strong if you left it stacked too high. They recommended block sanding until the primer starts peeking through the hight spots to make sure you had the surface as tight to the body as possible. Made sense to me.

I have only painted one car before, way back in the days of lacquer. My wife's 914. It looked nice, but being lacquer, was only temporary.

It was definitely easier spraying lacquer back in the day... more forgiving.

On the K36, I start with 180 or 220 dry in an orbital and/or straight line sander, just to get it somewhat smooth... because K36 is a lot harder than the old lacquer primer surfacer. Then I wet block sand with 220, then 400, then 600. K36 will actually sheen a bit with 600 wet. K36 spec sheet says 4 hours to sand after spray.

I found that using no reducer in the first coats is OK. But highly recommend a 5:1:1 ratio with reducer on final K36 coats. It was less splotchy when I tried my old siphon gun once, but overspray was pretty bad. So I reverted back to the HVLP.

The reason I use black DP90LF is that I can see it easier if I start to sand through the K36. And I also use the DP90LF on almost everything on the car that should be black... like suspension, rockers, engine lid, etc.

Today is sanding day on the hoods, engine lid, and eyebrows... with a small bit of filler here and there. Good luck!

Bumping this up since it posted over a holiday weekend. I'm now interested specifically in shrink time for K36 primer surfacer. I know it can be sanded after 4 hours of air dry, but being a urethane primer, there is going to be some shrink over time. Would a week in the sun be enough time to allow this primer to shrink back before blocking and top coat?

Also, how much primer surfacer do folks typically use on a 914. Planning to buy some this afternoon.

Thanks!

No idea of shrink time, and I've never worried about it. This is my 4th car spraying with K36... never had a problem. I also sand it until I see the black high spots.

Amount depends on what you spray. 3 quarts is plenty for the main exterior body.

High build primer surfacer should never be used as/like a bondo, the thicker the primer is in an area the more issues you will have with shrinkage. The filler work should be as close as possible to the finished surface, all pinholes filled, etc.
Only then should the high build primer surfacer be used for final block sanding..

This. This right here. If you arent sanding 90% of it off, you're not using it right.

kind of sort of.. I cant tell you that " bondo" shrinks just as much as primer surfacer.. I have been a home hobbyist painter since I was 20 or so and previous I did all of the prep work on my cars at my uncles body shop under his leadership.. I believe in the proper way and that is as others said as close as possible but the primer surfacer should be sprayed over everything,blocked, then shoot again and let sit for the winter to shrink etc then block in spring for final paint. So a 4-6 month shrink time will truely get you the best results..

Okay, so the people complaining about shrinking on the Interwebs probably are abusing the product. I was beginning to suspect that. I've seen some people doing meticulous restorations recommending letting the car sit for a month or more after spraying surfacer which is really what got me worried. I guess if you can afford that kind of time, there is no harm in it.

I'll still try to let mine sit a week before blocking. I've already applied a skim coat of high quality filler on top of DPLF epoxy primer over the exterior and blocked it all down until the high spots were peeking through, so the body should be pretty straight at this point.. I'll hit it with another coat of DPLF and let that cure for a day or two before spraying the K36.

And @roblav1 , thanks for the info on quantity. I'll pick up a gallon early this week because that will actually be cheaper than buying 3 quarts.

Thanks all for the help!