sand blasting

we use waxoil here in the uk for cavities, door shells etc. they supply a long tube with a radial nozzle on the end. you push it all the way in, apply pressure and slowly pull it back out coating all around at the same time.
neil.
https://www.amazon.co.uk/Waxoyl-6141711-High-Pressure-Sprayer/dp/B019CXRV16?tag=fabo03-20