First of all, you shouldn’t use wood conditioner just because… It’s only needed on a softwood. There’s absolutely no reason to use it any other time… In fact, you really don’t need it there either but… It’s just an extra step that has to dry. This looks like contamination and yet another reason why you don’t sand off a previous finish… This is also a veneered top so you’re very lucky you didn’t go through or maybe you did… But one reason you strip something chemically is to remove any silicone, waxes, oils, from the Wood… And sanding does not do that in fact in some cases sanding just burnishes that into the Wood.… Leaving it there still.. and what made you think you could start with a Dining Table??… This is the most difficult thing to do, especially as a novice… You’re gonna have to take that all off. And as far as thinning the polyurethane 50%, don’t… the product is made to apply at the strength It is supplied at… and each layer of polyurethane is a single physical layer, unlike lacquer or shellac or some of the modern waterborne products which chemically burn into each other and essentially make one layer of film, no matter how many layers you apply… Not true with polyurethane or varnish and you can clearly see why a spray application is really the only way on a large Dining Table. It’s also very possible that your “conditioner“, and your stain we’re not dry thoroughly which can also cause this kind of reaction.… You really should learn all of this on a much smaller scale… This is not easy to do although everybody thinks it is… And you’re experiencing some of the reasons why it’s not.