Its a type of common problem.
Go on coating, 1st coat needs a light sanding. Then apply a 2nd coat. Bubbles will form in 2nd coat also but hopefully they should disappear on drying. If not, repeat the process. The bubbles are comming from gasses escaping from a reaction in the wood. 1st coat should stop that. If not, 2nd coat should, ect. Depends largely upon wood species. Anyway, more coats look better!