Vinyl Polysiloxane and Polyether impression materials are stiff enough that material that locks under a bridge pontic or through a large gingival embrasure can make removal of the impression from the mouth difficult to near impossible. On the other hand, Alginate used in the same situations can tear easily or be dislodged from the tray causing distortion that renders the resulting model inaccurate.
Over the years I have looked for and tried multiple ways to block out these areas and prevent impression material from flowing into them. I have tried different kinds of wax that includes rope wax to more sticky or tacky types of periphery waxes.
The challenge is the wax does not stay in these areas and you are struggling to keep it in place and manage inserting the impression. Any type of silicone material for block out will bond to silicone impression material creating the same issue of the impression lacking in.
Recently I have come upon a great way to block out these undercuts, that works simply and predictably and does not interfere with the accuracy of the impression.
I have my assistant cut me 1 inch long strips of Teflon tape. We cut the Teflon tape into strips and place 3-4 in small sterilization pouches, sterilize and keep in our restorative tubs for chair-side use. I thoroughly dry the embrasure or underside of a bridge pontic which allows the stickiness of the Teflon tape to adhere to the teeth or restoration. I then insert the end of the Teflon tape with a cotton pliers, and condense the Teflon tape into the space with the cotton pliers creating a fairly dense pack.
The Teflon tape takes on the shape of the space and almost becomes solid when packed tightly enough. I do not need to block the space from both buccal and lingual as the goal is simply to prevent material from locking underneath. The Teflon tape block out now stays in place well enough that I can focus on the impression and managing placement of the impression tray. The Teflon tape may come out in the impression or remain behind, but if you are going to take additional impressions check to see if you need to add or adjust the block out.