Smooth PLA Prints

I've been experimenting with methods to smooth away the print lines.

There are some chemical methods available for ABS but the equivalent for PLA uses toxic materials so I'm not keen on that.

I've tried Hi Build car primer but it took so many coats and so much sanding that I lost more detail than I was happy with. This time I have tried three different solutions. Resin, normal nail varnish and household undercoat.

Nail varnish

Resin

Undercoat

I'll jump ahead to the conclusion. None of them was ideal for the models I am making but all of them had some uses.

XTC-3D Resin

I tried two resins. z-poxy Zap 30 and a resin specifically marketed for the job, Smooth-On XTC-3D. The Zap 30 is a bit too thick and cannot be applied so easily so that leaves the XTC-3D.

If you are going to use XTC-3D I strongly recommend watching the following video first. It's a bit long but has some useful tips.
https://www.youtube.com/watch?v=TPHTPQchO1M

The most important take out from the video is to spread the mixed resin thinly over a dish. I found that the working time was inadequate at less than 5 minutes from the mixing pot but using the thin layer in a dish it is considerably longer.

Space in the dish for the resin to spread out

Where I applied the resin while it was still nice and runny it worked well.

It is difficult to get right. Even when it is thin it does not always apply with an even thickness. Some areas in my test sample had insufficient resin but I can work on my technique to improve on that.

Resin was already starting to cure!

It is important to stop using the resin the moment it starts to go off. The thickening resin ruins the areas it is applied to, see the above photo!

For my purposes the resin needs to set horizontally to self level. The vertical areas pooled and had less satisfactory results.

Carve out the lost detail

The resin sets in a few hours but I prefer to leave it overnight to be sure. After about 12 hours it was rock solid. When cured it is hard and difficult to sand. I had to use a Dremel rotary tool with a grind stone bit to make much impact. Being so hard it is very time consuming to sand any tight areas without damaging some of the detail! Large flat areas are OK to work on with the rotary tool.

I tried various techniques on the finished resin but the resin is at it's best if it can be applied in one coat and left as is.

Resin took some sanding

It took a lot of care to avoid the resin flowing in to small holes and shut lines where I did not want it! After a few goes I was able to achieve the finish I was after but there was a lot of wasted resin. I could only apply one surface at a time which had to be left horizontal to self level. That gave good results.

Resin filled the detail on the side of the wing

If there was any fine detail on a face the resin would pull towards it and curve upwards or flow in. This was like shaving off 0.5mm from the detail. Not ideal but with care could be worked with.

Resin filled the steps on the bonnet

If I was making an organic shape or there was no detail, the resin would be ideal as it is just one step to a smooth surface.

Excellent on the bonnet but had to avoid the detail

With care and while the resin is still runny I have managed to get some good results on the bonnet that would have otherwise been very time consuming to achieve.


Nail Varnish

I tried two different types of nail varnish. A clear top coat and a coloured layer.

The coloured nail varnish worked well on small areas. It had the advantage of showing where I had used it and the thicker coating filled the thin layers exactly as I had hoped. Even with the brush that came with the nail varnish, I had enough control to avoid getting it in any of the small details.

Nail varnish on the door

Nail varnish dries very quickly so I was able to do three coats in only a few hours and could sand it before the end of the day. It was easy to sand so I was able to avoid too much loss of detail.

It did not work for large surfaces. Firstly the fumes started getting to me when using this much nail varnish, for me that eventually made me avoid using it. On the cosmetic finish, the three layers of uneven brush strokes ended up with the large panels looking like patchworks. This was difficult to get to a nice smooth surface. On the large panels I had to use a Dremel.

The clear top coat was not as good on the layers but I could use it to fill the fine pattern on perfectly horizontal surfaces where the coloured varnish would be unnecessarily thick.

Nail varnish did not fill the steps

The nail varnish was not thick enough for stepped areas, perhaps 10 or more coats would do it but I did not try that. Back to the resin for those parts of the model.

Undercoat

This is normal household undercoat. It looks to be a bit thicker than the nail varnish.

I applied 3 coats of the household undercoat. It dried thinner than I expected and I had to apply it generously to get it to fill the grooves. It did not self level and resulted in an uneven finish but not much different to the brush strokes with the nail varnish.

Needs more sanding to get even

It takes a few hours to dry between coats and I ended up leaving it a day each time. The result is initially a little disappointing and it needed a lot of sanding to improve things. I was removing more detail than I would have ideally liked before the undercoat got anywhere near flat!

The undercoat was easier to apply than the nail varnish, partly because of the much larger pot but had similar results.

Acrylic Primer

One last test was to check how they took paint. I was sure the resin would be OK, fairly sure about the undercoat but I had my doubts about the cellulose based nail varnish. I let each dry over night before spraying on the acrylic primer.

Nail varnish on the front half of the door works well

Resin on the far half of the bonnet fills the step lines

I needn't have worried about the primer. The resin, the undercoat and the nail varnish worked well with no undesirable reactions to the paint.


Conclusion

None of them are perfect.

The best surface finish by far was the XTC-3D but that lost a little detail. It would be the only one to use on car body panels to get a new car dent free result.

Nail varnish on one undercoat on the other

For areas round detail either the nail varnish or the undercoat would be OK but I tended towards the undercoat because it was in a bigger pot and did not have the problem with excessive fumes.
Either take lots of sanding to finish the job. That is difficult in some areas of the tiny models.