Painting skirting white

Hi
I want to change our interior softwood skirting from mahogany to white. What is the best undercoat to use. I am not concerned about the cost of paint but would like to have it looking nice without having to repeat a coat after coat. I'm hoping for a one coat finish after the undercoat has been applied. Any tips or advice appreciated

Thanks

Hi, I went from brown varnish to white so pretty similar to you.

I used two coats of stain block and then satinwood. I probably could have got away with one coat but wanted to make sure. Two years on and there's still no sign of the old varnish.