I just did something similar. The finish I achieved wasn't brilliant (mainly due to my lack of expertise I would imagine rather than the method)
Make sure the welds are below the normal surface of the surrounding plastic, then sand, bodyfiller, etc. The repair I did to the fairing came out a lot better than the small dents in the tank I tried to fix. :'(
Thanks ChristoT,
I was thinking more about the plain matt plastic finish rather than actual bodywork.
I'm uncertain what to do about the exterior damage as I have the DRYC1 (deep reddish yellow cocktail 1) which seems to be darned expensive and needs a basecoat for a proper finish (and I'm an appalling sprayer).
Opinions are like A**holes, Everyone has one. Some people seem to have more than one though which is a bit odd.