There is a din of beis hastarim also by tevilas celim…Someone has a challah board that is wood with silver then on top of it is attached a big piece of glass. However there is a small space in between the glass and the board. When immersed into water, water will go into the space between the board and the glass however not completely. Perhaps if held there for a long amount of time water would go in but not for sure. How is that space in between looked at it. Could that be considered beis hastarim that the water doesn’t actually have to get to there. Or maybe even though it’s not considered beis hastarim and that water does have to be able to get inside there however since the glass and wood are attached we look at it as one cli. When someone went to tovel their challah board they realized this issue and want to know what the halacha is. To take it apart however and tovel only the the glass or 2 parts would seemingly be b’dieved. However let’s assume that it can’t come apart, so in that case what is the halacha when there is this small space that water will not reach to every part.
Because one can’t get to the part under the glass without unscrewing it, and this is not part of the regular function of the board, the assumption is that it is considered beis ha-setarim, and therefore there is no need for water to reach it. It is therefore sufficient to immerse the board as is, without further action.
Note that it seems that the board will not require tevilah with a berachah. If we consider it a wooden board, coated with glass, the halachah is that it will require tevilah without a berachah. See Tevilas Keilim Chap. 1 nos. 5-6.