Useful tools to have and filler question?

Regards tools, a good pair of flush cutting side cutters would be a worthwhile investment, Xuron offer the ultimate in these & they are also less expensive reasonable ones available. They take most of the work out of cleaning up parts removed from the sprue & can save damage on smaller items (when removing from the sprue)

Wifey's nail files are also a usefully thing to have about & are dirt cheap, same goes for regular wet n dry paper.

I have also suffered from filling problems, but after trying lots of different brands, I now use 3M Acryl Green. I think 3M Acryl Blue is better, but it isn't available here in the UK. The stuff comes in a large 600gm tube, isn't expensive, can be thinned down if required & sands well.

Tamiya filler putty has less 'bite' in other words it won't melt the plastic & destroy the details. Before it sets the extra can be wiped off with a Q-tip dipped in Acetone (finger nail polish remover) If you need a longer set time thin it with liquid cement (Testors jar type) The trade off is it will 'bite' more and take longer to set. Small areas? The edge of a curved hobby blade or tooth pick, some even use thinned white (Elmers) glue to run it into the small gap. Automotive scratch filler has similar properties to hobby filler, stay away from using it for large areas. Millput is a two part epoxy that sets hard like a rock, it has a long set time and will 'bite' into plastic. If its strength you need then try it. Also scrap plastic or styrene shapes & lengths makes good filler, get a close fit and use liquid solvent cement to apply.

Practice will be the best lesson to know which technique will work for each situation.