So for some reason, not too many studies have been done on bathing the body in poisons. I think the real issue is that its just too hard to get undergraduates to agree to play test subject!
In general, I'd say you're in a bad place if the spider venom touches mucous membranes. The skin is an amazing rejecter of large organic molecules, mostly thanks to its outer layer, the stratum corneum. This layer is weaker in those membranes and the potential for diffusion into the rest of the body is higher. Accordingly, I would say you should avoid pouring over the face or taking a bath. Besides it being outrageously expensive to get that much venom in the first place, it is likely to expose the sensitive parts of the skin that would let some poison in. Given the ridiculous overdose involved there, even a little poison would be a bad news.
Pouring something over your hand might be more reasonable. We can pour all sorts of odd things over our hands if we're bored enough of life. Not everything, but some are surprising
I haven't found a general rule for what sized particle is rejected by the skin, but I did find an article on Hydraluronic acids, which showed that low molecular weight HAs (20-300kDa) passed through the skin while higher molecular weight HAs (1000-1400 kDa) had little to no permeability.
Our first goto spider is the Latrodectus, the widows. Their toxin is known as Latrotoxin, containing many protiens of which α-LTX, a high molecular weight organic toxin weighing in at 520kDa (130kA tetramerized), is the most toxic. As such, it is unlikely to get through the skin quickly. However, there are many lower molecular weight molecules in the venom that are not yet fully understood. It is believed some of them affect the permeability of the membranes that Latrotoxin attacks, but it is not known whether it will affect the skin. At the absolute least, I'd highly recommend washing your hands after pouring the venom over them... and honestly, I'd recommend not pouring venom on one's hands in the first place. It just seems like a reasonable safety precaution!
This being said, widows seem like they would have an especially bad secondary effect. Latrodectism is the name given to the effects of a widow bite beyond the local effects. The venom causes the release of large amounts of acetylcholine, norephedrine, and GABA, which can have systemic effects. While you might not suffer the localized effects of the bites, the accumulation of poisons from the entire affected area may lead to these effects.
Phoneutria, the Banana spider, uses PhTx3, a broad spectrum calcium blocker. PhTx3-4 is much smaller than α-LTX, weighing in at 8.5kDa. This tiny little beast would certainly slip past the skin and ruin your day with its LD50 of 134 µg/kg. Your only hope would be if it were to accidentally bind to non-neuron cells on the way in, but nothing I've seen suggests this would be the case.
Cheiracanthium is tricky to make statements about because we're not 100% certain about the full mechanisms of the venom. This article suggests the primary toxic component, CpTx1 is a 15kDa protein, so it should have reasonable success working its way through the skin. From there, it's just a nasty sort of thing. It has cytoxic tendencies which some say lead to necrosis. This might be the best case for your friend pouring venom over their hand: their hand simply dies. It also shows the ability to permanently depolarize muscle tissues, leading to a permanent contraction of the affected muscles. This does not seem like a desirable way to go. In fact, it is what happens when you contract Tetanus, which is considered to be a contender for one of the more painful ways to go.
So of all of them, none are a good idea to pour over you. Just don't. However, of all of them, the black widow poison is likely the most benign due to its high molecular weight. Of course, you specified undamaged skin. A little razor burn or a small cut would drastically change the outcome of this decidedly foolish experiment!
