I've been searching for the best way to smooth the print lines on my prints. I believe I have found THE answer.
I experimented with several things before I found a good solution.

First attempt: standard clear nail polish.
Pros: This works well. It brushes on easily. Does not deform the part. Does not require sanding. Comes with it's own applicator brush
Cons: Requires multiple coats. Smells horrible. Expensive even at the dollar store. I'm told it does not last well. Does not fill gaps

Second attempt: Two part 15 minute epoxy (ZPoxy).
Pros: Fills the gaps easily. Adds strength to the finished part.
Cons: Deforms the part unless applied very carefully (drips). Long-ish drying time. Requires sanding. Requires measuring and mixing. Destroys applicator.

Third attempt: Bondo spot putty
Pros: Fills small gaps well. Dries quickly. Requires little or no sanding. Does not stink
Cons: Can still require a sanding on a very poor print.

The winner is Bondo 907 Spot/Glazing putty!
This was designed for the auto repair industry to repair small imperfections and it does that very well on 3d Prints as well.
It's the consistency of tooth paste. You apply it by wiping it on the part with a paper towel.
The paste fills the gaps and dries within approximately ten minutes.
The paste is red in color so you will need to paint as usual.
It may require some light sanding with finishing sand paper.
A 4.5 ounce tube is about $6.50 mail order ( http://www.amazon.com/Bondo-907-Glaz.../dp/B0002JM8PY )
or about $9.50 at a local auto parts shop.


Here's a print I experimented with. The part is white PLA. The red is where the bondo was wiped on. If you look closely you can see the texture is completely filled in:

BondoApplied.jpg

The product looks like this:
Bondo907.jpg

I have no financial interest in any of these products.
If you find something better please let me know. I'm always happy to find a better way.

Jay185