[snip]
I've used Ronseal fence/shed preserver on old fencing. I've found it needed
a good three coats to get a uniform colour and you will probably still see
at least dark patches where the old woodstain was.

I found the first coat soaked in as if painting a sponge, the second was
somewhat better and the third went on like one might expect. I did give
each coat some time to dry before starting the next one. I gave up at this
point since I'd reached the desired colour.

If your friend has a panel part hidden, they could always try a small patch
and see how it goes.

Paul DS.