I get Minwax polyurethane and it is oil based, indoor use. But I have some projects that will be used outside, couldn't I use the stuff I have without having to buy spar urethane? I am thinking it will work because it is oil-based. Water wouldn't be able to remove it very easily. So why is it indoor use only?
regular PU is not well-suited to the outdoors because:
it has no UV degradation inhibitors in it, so eventually the sun will eat it.
it is fairly rigid/brittle. seasonal movement will cause it to crumble.
spar PU/varnish has UV inhibitors in it and it is more flexible than regular PU to prevent exactly those two types of failure.