My front door has been warping during winter which makes it very difficult to lock depending on the temperature.
I did some searching around and it seems that happens when the top and bottom of the front door isn't sealed and moisture gets in. Checked mine it looks like it's just wood with no paint on it, feels a little rough too.
What's the best/cost effective way to seal it? I've read about using oil based primer + paint, but there also seems to be 4-in-1s available etc at Bunnings.
Either use a primer then paint otherwise most paint nowadays has a primer in it
Blu Tack
Assuming you're an owner-occupier, make sure it's dry then paint with water-based paint (unless you prefer oil-based). If the bare wood is an aesthetic thing use a clear coat outdoor finish (e.g. Sikkens).
Outdoor paint. Most of them are water based acrylic and require no primer. You'll need to dry the door out first.
If it's time for a new door anyway you could put longer screws in the hinges and the latch plate while at it to make it more secure against being kicked down.
Doesn't need to be a new door to do that.
Weather strips for doors. Available at bunnings, flexible so they'll work year round