Bedroom & Bathroom Window Frames: Refinish without Stripping?

[Macca Wacca]

I'm currently refinishing two internal window frames, one in my bedroom and another in the bathroom. The bedroom window ledge (above the radiator) has lost its finish, while the bathroom window ledge has the bare wood exposed in the crevices.

I started stripping the bedroom frame with Home Strip Paint & Varnish Remover, but it hasn't been very effective. Three overnight applications covered with cling film haven't made much progress, although some areas are stripped. Considering the positive reviews, I'm thinking of trying Paint Panther.

However, I'm now wondering if stripping everything back is really necessary. Ideally, I'd like to keep the existing wood color and simply add a protective coat.

For reference, I've already purchased Sadolin Polyurethane Extra Durable Varnish in Satin and Rustins Wood Dye Red Mahogany, which is close the to the original colour of the windows.

Would appreciate any advice on whether I should strip the frames completely or if there's a way to refinish them while keeping the current color.

Thanks in advance!