I am finishing a coffee table with Polyurethane. Used it many times, but have never managed to completely eradicate those pesky brush marks on the final finish, despite sanding between coats. So this time I am considering using the rub on method. Obviously being a coffee table it's going to get heavy use, so would need many many coats. Can anyone tell me if I can do the first couple of coats using a brush, sanding between coats to get it smooth, and then switch to the rub on method with thinned poly to get a perfect finish?
I wouldn't recommend using a bristle brush at all. Use either a foam brush or a lambskin applicator. I don't really see a reason to switch between the two, but it should be ok. Make sure you follow the manufacturers recommendations. You do not want to begin sanding until the solvents have evaporated and it begins curing. Also, time between coats has an upper limit. See the manufacturers website.
Two additional thoughts:
You can use a thinner such as mineral spirits to thin out the polyurethane. You will need more coats, but it will help eliminate stroke marks.
If you really think a small application like a coffee table will take that much abuse, you could use "Glaze coat". It is a two part that goes on extremely thick and works very well. Commercial bar tops and table tops often use it. It's not cheap. Last time I used it I paid $25 for 9SF of coverage, but it goes on smooth and when you are done you could practically weld on it ("slight exaggeration")
I use very expensive brushes with poly and always have good results, I am talking brushes in the 15-20$ range. I have never had much luck with anything else. After done I clean most of the poly out then put the brush in a jar full of spirits and seal until next time. The problem I see from DIY folks is they try two hard to work the poly it starts to dry and the brush strokes stay. I prefer brushing over rubbing rubbing will take 5x the coats to get the same depth look. I would work on your method of application, no more than 3 strokes reload and apply more. Thinning down the poly can make applications easier but you need more coats. Don't go cheap on your brushes this has a large impact on how the poly is evenly distributed.