If you want to achieve a ‘smooth as glass’ finish on open grained woods, then you really need to use a grain (aka pore) filler. Woods such as oak, ash, elm, mahogany, chestnut, walnut, wenge, and teak are characterized as having ‘open grain’ because the wood pores are large.
In contrast, ‘closed grain’ woods like hard maple, cherry, poplar, beech, and bubinga, have much smaller pores.
A grain filler levels out the surface of open grained woods by filling the pores, reducing the number of topcoats (and the requisite sanding between each coat) that you need to apply to achieve that smooth finish.
Aqua Coat is a superior water based grain filler that dries super clear.