No matter how rough your print turns out, it is possible to give it an automotive finish!
The picture below shows an extreme example of a rough part before and after my finishing process.
This is my technique that I use on ABS parts. I don’t like the acetone methods for many reasons so I do this instead. It sounds like a lot of work when you read through it but in practice it goes quick and looks amazing when done. Just don’t get carried away with any of the steps and you will be fine.
Straight off the print bed, once you remove the bulk of any flashing and or support materials, the first step will be to nock off any really big stuff with a rough sandpaper (80-120 grit) or a file (get a nice hobby set). DON’T TRY TO SMOOTH OUT PRINT LINES AT THIS TIME!!!!
This next step begins as an evaluation of the part. If there are any major flaws (pinholes, gaps, etc), use a filler putty to fill them in. Don’t fall for the “two part bondo is better” trap. On things of this scale, a squeeze tube of a one part filler is far easier to work with. I use Bondo brand glazing putty #3. Glazing putty Just a tiny amount on the trouble spot of the print at a time. If it is a large imperfection, you may need to apply the putty, let it dry, and apply a second coat. Don’t blob it on. And again, DON’T TRY TO SMOOTH OUT PRINT LINES AT THIS TIME!!! Let it dry for a few hours before moving on.
OK, now grab another piece of sandpaper, 250 grit, and smooth out the spots you put the filler on. Be gentle in your sanding as it won’t take much if you used the putty sparingly.
This pic shows a very rough part coated in several thin layers of glazing putty to allow some shipping to be done.
In this picture, I put on a thin coat and started smoothing but it still needs more putty. The dark areas are where the sandpaper is not touching the putty. Don’t force the sandpaper into these indentations. Instead, use them as a guide to where your next round of putty goes.
After a thin layer was added to the part in the above picture, I sanded again. Below is the result. A few dark areas remain but it is almost smooth.
This shows a part that had some pretty heavy print lines. The glazing putty was not necessary to smooth them out and on the other side i only used filler primer, which saved lots of time. The reason I did this was more as a visual aid. Use your fingers to determine that a part is smooth, not just your eyes. This picture illustrates a very smooth part that to the eye, looks rough.
Wipe the part down with a clean damp lint free cloth. Now we get to take care of those pesky print lines. At the local auto parts store, they sell something called “filler primer”. It comes in a spray can and is usually about $5-6. Duplicolor, pictured below, makes a decent filler primer but I am preferring the Rustoleum brand right now.
Apply a decent coat to the entire part. It may take a bit of practice to get a nice coat that is thick enough to start filling the print lines but does not blob or cause runs. Your best bet is to go a little light. I can usually get rid of all lines in three coats. You don’t have to sand between coats but I do recommend it if you are going for a true automotive finish. If so, a good 250 grit sandpaper should do it. Make sure to let each coat dry for several hours between coats if you sand, otherwise an hour should be fie. Also wipe the dust off with your damp cloth each time.
So now your print lines are gone but the work is not done. Give that final coat a sanding with a 600-800 grit paper, then wipe clean. Now get the colored paint of choice and apply light coats as you did with the primer. You will want to switch to a wet sanding 1000-1500 grit at his point. Once you have a really good finish after a few coats, you can hit the part with a clear coat if desired. I recommend this to help protect your now beautiful part.
If you want a brushed metal look, use a gray filler primer, then get silver rub n buff and apply it with a lint free cloth. An old sock works great. Just rub on, then use a clean section of the cloth to buff to desired shine”