Bed hardwood window board on mastic? Or similar

[Albumen]

I've got a hardwood windowboard for the kitchen. It's going to be going on to a flat plastered cill.

In the past in this kitchen we've had problems with silver fish, the little devils. They were living under the tiles, which had be dot and dabbed. I want to fix this window board with no void under it at all, however small. In the bathroom upstairs I fixed it onto wet sand and cement and screwed down tight. It warped a bit, surprise surprise.

I thought this time I would fix it onto some sort of mastic type material. Spread a thin layer onto the plastered cill, with a notched tiling trowel (front to back so the air can get out), then screw down hard, hopefully leaving no voids.

Is there a mastic/glue/sealant that anybody would recommend?

Thanks.

 

[foxhole]

No nails.
If screwing don't screw through centre of boards as it will still allow bowing.

 

[pinenot]

Gripfill, does as it says in the name, grips and also fills.
If you want to spread it with a notched trowel, I would omit using any nozzle pump out large strips, along the length of the plasterboard cill, then trowel this and fix down the wooden cill...pinenot