Paint Application: Sanding primer coat before applying topcoat

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
bobross
Posts: 9
Joined: Fri Oct 30, 2020 6:32 pm

Paint Application: Sanding primer coat before applying topcoat

Postby bobross » Sun Dec 27, 2020 7:58 pm

I have just finished applying the primer coat [of paint on a hull being restored]. The primer coat has a pebbly finish.

Q: Should I sand [the primer coat surface to be] smoother before applying the topcoat?

I read--maybe on the directions of the Wet Edge topcoat paint--that you should sand the primer smooth with 220 grit before applying the topcoat.

Please offer advice.

Jefecinco
Posts: 1193
Joined: Fri Oct 09, 2015 6:35 pm
Location: Gulf Shores, AL

Re: Paint Application: Sanding primer coat before applying topcoat

Postby Jefecinco » Mon Dec 28, 2020 10:02 am

Did you apply the primer with a spray gun? Could the result be "orange peel" in appearance? I would want a perfectly smooth final primer coat before applying finish coats. If sanding the primer smooth removes too much of it and you need to reapply it consider changes to your application method. If you are using a roller the nap of the roller can make a significant difference in the outcome. The thickness of the application may also contribute regardless of the method used. Environmental conditions can also affect paint application outcomes. Recommend you carefully read the instructions, if any, that came with your paint and do further online research on the subject before proceeding.

There are some experienced painters on this site and I'm sure they will provide some expertise.
Butch

dtmackey
Posts: 467
Joined: Thu Sep 28, 2017 9:29 pm

Re: Paint Application: Sanding primer coat before applying topcoat

Postby dtmackey » Mon Dec 28, 2020 11:13 am

I'm assuming you primed with an epoxy primer as they are usually the best for building a great base that adheres to your topcoat. On my projects, I sand the primer with anywhere from 180 grit to 320 grit depending on how the surface looks and the level of finish I'm looking for. There are many factors that play into this:
  • type and brand of primer
  • type and brand of topcoat
  • how the surface is sanded, that is
    • by machine
    • by hand
    • by wet sanding
    • by dry sanding
If you sand with 180-grit, then bump up to 320-grit, then I feel the [top coat appearance] should be fine--but always perform a test on a small area to make sure you achieved the surface preparation [that will produce the top coat appearance] desired.

D-

jamesnovak
Posts: 14
Joined: Sun Aug 30, 2020 3:23 pm

Re: Paint Application: Sanding primer coat before applying topcoat

Postby jamesnovak » Wed Jan 13, 2021 9:09 pm

I applied a top coat of paint with a foam roller to a primer coat without sanding the primer coat smooth. Predictably, the top coat paint ended up being bumpy as well. The top coat paint goes on very thin, so I'd recommend getting the primer coat as smooth as possible with a 220 & then 320 grit sandpaper.