Powder coating is pretty thick stuff,if you got any on the internal bore,it would affect bore-size.Also bear in mind that the powder is usually a polyester resin that is statically charged prior to appliction and then cooked on and so is no way as hard anodising.There are reasons why anodising is effective on aluminium and so is the prefered coating medium- the hardness,colours that are available and it being cheap in large batches.