Hi!
Polyester resin and brown microballoons (plastic) are best for filling dents and holes on our airplanes and boats. White microballoons are made of glass and are harder to sand.

The soft, light hobby filler you can get at hobby shops is no good as thse are very easy to sand but will leave a soft surface which is no good for if you want a nice surface on which to put automobile primer and put on a high gloss paint on.

You should never use 5min or any other glue epoxy when using microballoons as the mix will be very hard to sand. Using polyester resin and brown microballoons is very easy to sand and you see the difference in surface structure where you have sand and where you have not.

Even better than polyester resin is to use 24 hour mold making /finnishing epoxy resin. Not the hobby stuff variety, West system has a rather good product but there are much better (harder more easy to sand) finnishing resins on the professional market.