preserving whitewood

If you think rot or woodwork is a real threat to it - ie you already have problems in the garage with either then treat it with two coats of the nasty clear smelly solvent based antirot and woodworm treatment and then maybe a coat of tough polyurethane varnish on top.

I would probably only bother doing the latter.

already

Rot would indicate rather high humidty or a non weather tight shed thus damp timbers. Worm, like rot, don't like dry timber. If the shed is dry I'd not expect either to be a problem and a coat of water based varnish will probably be anough to keep worm out, as the bettles won't like the surface when it comes to laying eggs.

IGBFY? (Is Google Broken For You?)

Cuprinol 5* fits that description (clear rot woodworm treatment) and pops up in the Shop For ... box on the google results page.

These or a rival formulation if their price is better.

I am loyal to ICI due to employee discount (declaring an interest).

Couldn't spot the high VOC trade only one, but I am fairly sure a Decorators Centre would have it.

You might also want to paint this stuff on anywhere that shows signs of having active woodworm/fungus and/or fumigate with pyrethrum smoke when it is the time of year that the adults fly (available from eBay/Amazon).

I recently asked for polyurethane in Wickes - they said they'd never heard of it. One guy was at least 50 and said he'd worked there for years. I walked out wondering which of us had gone mad.