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depending on how wide the slit
a thin stiff bit of Ti or scrap wood and put it in a plastic bag

dribble CA in the gap and then rub the plastic covered "stick" inthe gap to spread the CA out
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hand sand the inside and then a light coat of CA is how i do it (its more for the sealing of the wood inn mmy case

its not the deep smooth finish that you would get on the outside but its a nice...