I applied too much polyurethane finish on newly stained wood farm table while applying third coat. I tried sanding with 220 grit and applying a 4th coat but there are still very visible streaks. Should I sand and coat again or do I have to strip and stain over?
I would go to a finer grit after the 220. Yes you need to sand until it is SMOOTH. Do you have a random orbit sander? Wipe on poly (50/50 poly and thinner) is a good way to go. The coats are thinner so you need 2 or3 more. Brushing works well but with a GOOD brush or a foam brush.
I don't think you need to strip it completely. It sounds like you didn't get the last coat fully sanded off. How long are you waiting between coats? If you are sanding between coats, you have to wait 24 hours for the previous to fully dry/cure.
Personally, I prefer the wipe on poly, which is basically poly that is thinned down by 50% or so. It's much easier to get it smooth consistently.
Everything is fine. Don't worry about the small imperfections. Wait for the polyurethane to dry completely and cure as hard as possible.
Depending on the level of gloss you want, go to an auto supply store and get a bunch of different grits from 300 and up. Get an assortment pack and stick with 3m. Cheap sanding paper is more expensive, since they don't last anywhere as long as the premium ones.
Get a sanding block or make one. If you have a orbit or sheet sander, even better. Start with 220 and lightly sand the surface until it is evenly hazy. You should get a powdery dust on the surface. If it is gummy, then the poly isn't cured enough. Wait a few days before sanding.
Clean off the surface with a damp rag see if there is an even surface, If the poly is thick, you can sand again and if it not, then add another coat of poly.
Now, work your way up the grits and check the surface to see if there is an even sheen. If you like the sheen level, you can stop and buff it with wax. Continue with higher grits for a glossier finish.
If you want to prevent streaks, you should use a wipe on poly or foam brush.
Check out this article: http://homesteadfinishingproducts.com/rubbing-out-finishes/.
When I first tried to get a glossy finish on a piece of stained wood with Shellac I was getting streaks in my otherwise glossy finish because I had a hard and imperfect sanding block. My solution was to stop sanding at grit 400 or 500 (probably could have stopped at 320) and then just use really fine steel wool to finish rubbing out the finish after it had hardened for a day or so (remember to let the wood harden before rubbing it out ...). This was done as suggested in the aforementioned article.
If you have a nice cork sanding block or a random orbit sander as suggested by Lee, then you can probably just sand all the way to grit 1000 to get a really glossy finish. The steel wool will give you a more matte look, but I like it that way.
I wet sanded the shellac with soap and water, but left the finish dry for the steel wool. The article mentions that you may be able to dry sand with poly though.