Reddit mentions of Rust-Oleum 260510 Automotive 2 In 1 Filler and Sandable Primer Spray Paint, 12 oz, Gray

First off, your post isn't stupid. There is absolutely no shame in asking questions - that's the only way to learn, right? And you can absolutely paint it. I did mine and my dad's front doors last summer, and there's no fading or peeling of any kind after a year of sun and weather. But there's a trick to it: treat it like you would plastic or fiberglass automotive trim.

• First step (obviously) is to remove the hardware and get it outside on sawhorses.
  
• Scuff it with a red Scotch Brite pad, clean any dust off with tack cloth and wipe it with down with acetone.
  
• Next, spray it with adhesion promoter and follow that up with a quality spray primer. Keep in mind other manufacturers make these products, too... I'm just linking what I use (I use Rust-Oleum or 3M for my primer coats, then Krylon for my color and clear... I get better finish coats out of Krylon, but better surface prep with Rust-Oleum and 3M).
  
  Now for your next step, I need a little more information: are you just gonna go with spray paint or do you wanna brush on a latex? I went with spray paint and a satin clearcoat for mine, but I used a latex and put a faux wood grain on my dad's (which changes the following steps). Which route would you like to go?

For me high speed (Dremel/finishing sander) sanding on PLA only made things worse.

The plastic started to melt into the sandpaper, ruining the sandpaper quickly without accomplishing much.

If the details of the print can survive it, the best way to finish PLA is to hide everything under primer paint.

Hit the PLA with 120 and 220 sandpaper by hand. Light and slow are the operative words here--plus patience. 120 may not be needed or may even be counter-productive on 0.1/0.05mm parts.

Do a second pass with 400 grit. The part should be relatively smooth to the touch.

Then I apply filler primer (smaller details, not going to sand, don't want to smother) or sandable filler primer (flat surfaces, larger details, both details and surfaces can be sanded by hand).

Filler primer of either sort works best in many light coats rather than a few heavy applications.

Sand filler primer with 400 (or 220 if there's somehow too much of it). You can even step up to 800, 1000, 1500, and 2000 (I did this to ONE part). Again, light and slow or the sandpaper will quickly pick up paint that is now mashed in, ruining it. Don't bother hitting the PLA with anything above 400 grit--it doesn't work well and eats through sandpaper. You're better off using higher grits on the paint rather than the part.

Finally apply whatever paint or other colors you want on top of the primer.

I highly recommend a competent "shop-vac" to contain dust and lengthen the lifetime of your sandpaper. If you don't sand too "hard" most of the plastic/paint will vacuum right out, greatly extending the lifetime of each piece.

If you have absolutely no access to a CNC milling machine (such as a Shopbot) of appropriate size, disregard my comment. If you do, please do not follow the hobby tutorials about making a female mold from a male plug.


The female fiberglass molds made in this way suffer from systemic lack of stiffness simply due to being too thin and flexible. It is vastly preferable to have a "brick-like" mold with a carved-out layup surface than a "shell like" mold, both for workability reasons as well as quality issues that arise from doing essentially 3 layups by hand and the imprecise processes that accumulate into dimensional errors along the way.




Here is how I have done it.

https://www.ericpillai.space/blog/2018/11/11/compositepart1

The end result looks like this:

https://www.instagram.com/p/BwpraXIhN4Z/?hl=en

I have exclusively used MDF as a mold making material. It's not exactly the "best" material for molds for a variety of reasons, but it is more than sufficient for one-off parts. It is also very accessible, cost effective, and easy to machine/work with.

Prepping the mold for use is done in 3 steps:

1. Spray painting the mold surface with sandable fiberglass primer paint like this: https://www.amazon.com/dp/B006ZLQ4HQ/ref=cm_sw_r_cp_apa_i_1INmDbCTS0H2R
   
   
2. Sanding the inside to as smooth of a texture as is reasonably achievable
   
   
3. Coating the mold in mold release wax using a microfiber towel in 2 layers (let the first layer dry before putting on the 2nd): https://www.amazon.com/dp/B002J854E8/ref=cm_sw_r_cp_apa_i_TKNmDbR2EDJKX
   
   We have had very good results with this method, with a few caveats:
   
   
4. Your mold will be "hairy" once you pull out the part (MDF fibers lifted from the surface). At this point, it is basically unusable unless you wanted sand it down again to make a part that was 2% bigger in every dimension than the original.
   
   
5. If you don't do a good job letting the filler dry, sanding it, and applying wax, your finished part will transfer large paint flakes on the outside. Not terrible, but a pain to remove.
   
   Don't feel duty bound to any of the specific products I linked, these were just the ones we had on hand. Other equivalent brands are probably about as good or better.
   
   Also, getting a good surface finish will require post processing, there's no getting around it without a very very expensive mold. Just set aside an afternoon with you, a fresh box of sandpaper, and a nice spotify playlist if you want a mirror finish on the composite.

Yeah, it's pretty nice at filling the area between the print lines. I've only used rustoleum
and krylon (but I can't find a link to that one)
Basically it's just a process of sanding the material down a little bit until the layers aren't too bad, spraying on the primer and waiting for it to cure, then sand it down so the low areas kepp the filler primer, and repeat until you're happy

LINK

I like XTC-3D a lot... I don't know if you're like me and tend to sorta gloss over warning labels, but just in case - buy a respirator before you start sanding it, and definitely do it outdoors or something. That stuff is nasty.

I like Tamiya putty but mine always tends to dry out... I haven't used any of the other products you showed in there (thanks for doing that, by the way!!). Did you like the perfect plastic putty? I've never thought of trying the syringe thing before, might give that a shot...

One thing you can try using sometime is filler-primer (see here). It's softer than XTC-3D but it's really great for hiding the layer lines if your model isn't too detailed.

Edit: I just noticed the link to your video instructions for modeling, thanks for doing that as well! I've never heard of ZBrush, and I don't have $800, but I might watch anyway to see if I can extract some good tips for the modeling suites I do have access to. Thanks again! Also great work on Finn and F.P.