DIY painting disaster

[Frickssake]

Decided to paint kitchen windowsill. Lightly sanded it down - washed it, dried it, masked it off. Thought few coats of paint would fill this - but obviously hasn't. What do I do next please? TIA

You will have to sand it right down past these flaws, then smooth sand, maybe prime if down to the wood, a good undercoat and top coat

Large candle over it and poof it's gone

"Fine Surface Filler" from B&Q. Sand down again, and repaint. Paint itself is too thin to fill imperfections.

Was wondering ( friend has an electric sander ) to sand it down back to the wood but has anyone any ideas on how to stop/limit the mess a Sander makes please. ( It's in the kitchen - don't fancy dust everywhere)

Most modern sanders have a little bag that catches the dust.

Sand it , use paint stripper , try fill it or use s plane hmmmm

There isn't a truly reliable way of preventing dust when sanding. I have seen it done as a "two-person" job, with one person doing the actual sanding, and another with a vacuum-cleaner nozzle following up a few inches behind.
Whenever I have need to remove an entire coat of paint, (such as on a door or windowsill) I have used a chemical stripper to remove the actual paint, leaving a modest amount of sanding to be done at the end.
The modern chemical strippers do not smell as bad as the old ones used to.

Sand, fill and sand again then paint

I wouldn't use a plane. It requires some skill and experience, and unless it is a "Bull-Nosed Plane", it will not get into the corners.
I have used my electric power plane a lot, and the result is often "wavey".
Keep it simple-actually the simplest thing is to put things on the windowsill, and forget about it!

Sand, fill and sand again. Then gloss. Remember to sand in the one direction. Along the length of the sill in the direction of the grain. 🤙