Filling the Grain
Tight grained woods don’t need filling. I rarely fill. But oak and mahogany have larger gaps in the grain that will effect your finish.Test your finish first to see what color it produces. Then, go look for a filler the same color as your finished piece will be. Don’t fill with a color the same as the raw wood because filler doesn’t change color much after the finish is applied.
Use a rag or stiff paint brush to apply the paste filler. Work it into the grain and let it dry. Remove the excess filler with a plastic scraper or a smooth, round-edged putty knife. Sand lightly if needed.
Sanding sealer is like priming the wood. The sealer reduces the tendency of some woods to absorb stain unevenly. Sealing end-grain prevents the wood from absorbing too much stain and becoming darker than the rest of the surface.
You could also thin the finish you are using 50/50 with what is used to clean it up or thin. It should tell you on the back of the can.
Liberally apply the seally and let it soak in for a few minutes. Then remove excess with a rag. When it is dry, sand it with 320 no-fill paper or worn down 220.