I used to wash my shells - with hot water - washing up liquid and a scouring pad !
I have not 'scoured' the inside for years - but decided to try it again with my X-ray shell.
I used a Brillo pad (carefully) although ideally a green rough pad would be best.
This gives the inside of the shell a very dull - scratched look, so I was carefull to only 'scour' in one direction. This should really help the paint 'key' to the shell.
I recall people used to do this - but stopped in the main. I think I'll try and 'key' shells from now on as no matter what you do, they will chip!
Also - once its painted you do not see the scratches.
Some good quality fibreglass tape inside helps
