Polypropelene tanks/body work are not sealed to my knowledge. The tried and tested method of restoring injection moulded plastic parts is to use scraping to remove oxidised outer layer and deep scratching, then wet and dry in various grades to first knock down the high spots and then in the latter stages bring up the lustre. The final finish depends on how fine you go, but very careful use of a heat gun can work wonders as well, I guess this kind of seals the finish - in reality on a macro scale it is more likely cauterising the fine "fur" which is picked up by the abrasive steps and gives the nice shiney finish.
There are many trains of thought on this subject, Mr Codrington actually wrote an article for VMX mag a while back which is a great resource if you can get a copy.
