How can I restore the surface of ABS polymer products?

Recently I asked a question about [de-yellowing Lego](https://chemistry.stackexchange.com/questions/44309/de-yellowing-lego/44336?noredirect=1#comment73665_44336). I have a follow-up question on this. 


I try to design a setup where I can de-yellow Lego and remove/obscure any scratches from the surface of Lego items. 


Matt\_black mentioned it might not be possible to only fill scratches etc. 
With my limited knowledge I could imagine the next options: 


1. Dissolve ABS in a solvent, but this will be an unspecific process.
2. Heat treatment, but I think the items will warp due to internal stress.
3. Maybe the surface energy of a scratch is different and you could make a marker which only releases the polymer on the scratch region?
4. Coat the items in some kind of transparent resin?


My questions are:


* Are any of these ideas realistically feasible?
* If not, are there any other options which could work?
* How would this procedure work from a chemical point of view?


