I'm a fairly noob DIYer but I'd like to make the inside edge of one of the windows finished. It's the upper part of the window which is about 20cm by 40cm. So a relatively small section. Could I get small tub of polyfilla and cover and smooth it with that and then put on a couple of coats of paint? I know it's not gonna look perfect but will be a lot better than it is. Is there anything else I could do that would suit basic DIY skills?