I finished priming the hull bottom with Pre-Kote. Â I sanded with 120 grit between the first and second coats of primer. Â This brought me back to epoxy and fairing compound in a few places, but knocked down the high spots. Â The second coat of Precoat went on well with no major high or low spots. Â I will overcoat with three coats of Interlux Bightside in Flag Blue. Â My question is: must I sand again with 120 (or higher) grit to give some tooth to the Brightside, or is the primer fine to be overcoated as-is? Â Most primers for other uses require no sanding before a finish coat.
Dave
There may be a time period where you can add coats without sanding but I am not sure. Sort of like epoxy where if it is still green the second coat of epoxy will chemically bond to the first one. I know there is a period where you can apply the second overcoat. If you decide to sand between top coats I would lightly sand with 120 and then go to 150 or 200 between finish coats. One thing I found with Brightside paint is it isn't totally hard until after 5 days. That may be a day shorter in the heat but again I am not sure on that. When I tried sanding it after a day it gummed up the sand paper and messed up the coat badly. I had to sand off the entire layer in that area and reapply it.
Terry; Maybe the primer was a secondary primer, sprayed on glue to help prevent the finish coats of paint from running or sagging, very un-sandable as you know. What I do with auto painting is use sandable primer with very little thinner so it will "Fluff UP" and after 30 minutes wet sand by hand to fill in little imperfections, spray on the secondary primer (glue) then two or three finish coats of paint.
  If you go to your local NAPA or automotive store and buy 3M masking tape that's fresh (Squeeze it) you'll only have to mask once.
  Thanks again everyone, hope Im a bit helpfull in my own way.
      Alan
Hi Dave,
Primers can be directly over coated, but this usually doesn't produce the smoothest finish. If you over coat the primer within the "chemical window" of opportunity (6 - 12 hours with Pre-Kote), you'll have a good bond, but I rarely find home builders organized enough to move directly to top coats, after primer within the allotted time. I also dislike this practice with solvent based paints. I prefer to have a "hard cure" on the primer so out gassing and shrinkage aren't issues.
Couple this with the fact that these type of primers (Pre-Kote, etc.) have fillers in them to help at blocking color from under it, make a uniform base and more importantly offer something to sand. Pre-Kote uses phenol microspheres, other brands use balloons, talc, etc. This is just like thickened epoxy and it sands very easily, fills minor imperfections (the real reason why you sand between coats) and bulks up the paint.
If you roll and carefully tip off primer, it still will not look great. It'll have brush marks and thin areas which need to be leveled out a bit. Usually all you need to do is a few passes with the longboard and a finish grit. The subtlest of lows are filled, brush marks flattened or stipple smoothed and imperfections filled.
I use a lot of primer on fancy paint jobs. I start with a thick base of 3 - 4 coats of building primer. If the primer looks or feels thin, I'll add filler material. This gets "blocked" which is short hand for final surface filling, which removes at least half of what is put down, but the surface is smooth. Two more coats, lightly sanded between coats, then wait for a hard cure (a few days, depending on brand). The last coat is final sanded, sometimes wet with the finish grit just for a "tooth" so the top coat has something to grab onto. Naturally, this is a lot more then the back yard builder will usually do, but it produces a smooth surface that can accept very reflective top coats, that will not be ashamed to live there. Nothing looks worse then fancy LPU paint over swirl marks and obvious imperfections. These high gloss paints show everything. If brush strokes are visible, you can hide many things in them and using less then high gloss paint also helps hide imperfections well too.
As for the original question of sand or no sand, well I sand because it insures tooth. Sanding primer is an easy thing to do. I could scuff up for another coat on a Weekender size hull in 15 minutes.
Thanks, Paul. Great answer.
Dave
yeah thank's for that paul. i will be fairing and getting ready to paint my bottom soon, and I will use this method, i used to Paint cars (30 years ago) and We used differant colored primers as welll. so high and low spots were taken care of. but i did not think of the "filling quality" of primer.
So what you are saying is you lay down 4-6 coats of primer b4 you even begin the between coats sanding?