Sanding down gloss before applying a water-based satinwood paint

I need to sand down some white gloss sills and skirting before applying a water-based satinwood paint (Johnstones - which is self-undercoating). I've just spent about 15 minutes sanding down one window sill (with a medium grade - 70 - sandpaper), however, after rinsing it off, there is still a sheen on the paint - does every single bit of sheen have to be removed for the water-based satinwood paint to stick? Somebody suggested using a product by Bin Zinsser first, however, it is quite expensive and I can't stretch to that now after buying a new sofa, carpet, wallpaper, paint, etc. Does all of the sheen have to be removed, and/or is there a cheaper product than Bin Zinsser that I can use which means that you do not actually have to sand off the gloss paint? Thank you.