Penetrol has several products, but I've used the additive penetrol to Pettit's Captains
Varnish with better results then just the
varnish itself. The penetrol helps the varnish soak into the
teak better, which helps it last longer.
The problem with exterior teak is it's an oily
wood. So in hot direct sunlight (California) it wants to bleed out the
oil. Which in turn bubbles up under any coating you put over it. So the better the "Penetration" the longer it lasts.