Guess I have never heard of any method or anyone trying to preserve the tip paint but a couple thing come to mind when sealing something.
You could clear coat it with the spray you find at hobby shops...but being a chemical based spray like a laquer you may have a reaction to the paint if put on heavy.Spraying a ckear coat(both gloss or satin) you may have to almost dry sray it and let the layers build up.I have seen people clean trench art and after polishing it they clear coat it but never seen it used on tipped ammo
A wax/parafin may also seal it but may not give you the results you want
The product renassiance wax is a protectant and often has been mentioned on the forum.Its a thin micro crystaline wax made to coat all types of surfaces and a little goes a long ways.It wont harm the paint.Its recommended for most types of metals,wood,leather etc.If I was do do this I would use the renaissance wax.