Sandable primer suitable for PLA?

Question

What kind of primer should I be using for my PLA prints?

I want to be able to sand the object after applying the primer for a smooth finish before painting.

Would something like this work?

https://www.bunnings.com.au/dulux-duramax-325g-plastic-primer-spray-paint_p1400720

Answer 1

There are generally 2 types of operation you might want to do before priming your object:

• smoothing to a point that you are comfy with and
• roughing pass with very fine grit to give the primer something to stick to

Smoothing via Sanding & Filler

To smooth your object you generally have two options:

• Only sand down (and possibly vapor-smooth) and account for the lost size in the design phase.
  • Vapor-smoothing can flatten away surface details you might want to preserve, so it might not be an option at all.
  • While vapor smoothing is possible with more than just Acetone on ABS, the price tag for those chemicals is usually more expensive by a factor of 20 or more due to the fact that Acetone is pretty much dirt cheap as far as chemicals go - and easily accessible via home depot.
• Fill up with body filler in areas you want and sand smooth afterward.
  • This is very labor intensive, especially for complex shapes.

Only after you filled up the structure to be somewhat smooth you apply paint primer. In itself, common spray on primers often are not filling enough to hide away the printing layers.

For rough surfaces that need a starting fill, a paste body filler applied with a spatula works best, and for the last pass over a just lightly scratched surface, automotive body filler from a spray can works great. The benefit of spray-can body filler is, that it also acts as a first roughing step, so you don't need to roughen the surface for the primer. If you grab a filler-primer, even skip the primer.

Sidenote from experience:

Some filler-primer and lacquer spray cans seem to contain solvents that are able to soften PLA.

Smoothing via Coating

A random find on Thingiverse showed me another way to paint and flatten the surface faster and without sanding, at the cost of details getting smoothed away:

• Apply a thin paint coat.
• Apply a thin coat of a fast drying, transparent Polyurethane coating onto the wet paint.
• Let the combined layers dry with extra air flow to prevent noses.
• Repeat as needed.

It works by picking up the wet paint and embedding it into the thicker Polyurethane layer, which dries much smoother than the paint itself, filling up the imperfections and the steps between layers. This process will however also fill up non-masked surface details you might want to preserve.

Your product in mind

The primer you took a look at works on PLA, but it would not smooth out all the dimples. It might work nicely for the PU-Buildup variant.