If it were me, I would just buy a sheet of plexiglass at your local home center and put that up against the brick wall. Drill your holes through the plexiglass with a 1/8 inch twist drill bit, and then continue each hole into the mortar with a 1/8 inch masonary bit. Then remove the Plexiglas, and enlarge each 1/8 inch hole in the mortar joints with either a 3/16 or 1/4 inch masonary bit to accept a plastic anchor. Use 1 1/2 inch stainless steel sheet metal screws to mount the plexiglas with a rubber plumbing washer or nitrile O-ring under each screw head to keep the holes water tight.
If you do decide to seal the brick and mortar directly, then use a film forming ACRYLIC sealer like Tile Lab's "GLOSS Sealer & Finish" available at Home Depot or this one:
available from Glaze N' Seal in California.
Glaze N' Seal
The Glaze N' Seal product forms a very hard film that will last for many many years (my experience is 10 years or more). The Gloss Sealer & Finish is easily removable with Tile Lab's "Heavy Duty Grout Cleaner and Stripper".
The problem with all siloxanes, whether they be silicone caulk, silicone based grout sealer or any other kind of silicon based plastic is that they don't stick well to each other, and nothing sticks well to them. So, for example, silicone caulk won't stick well to silicone caulk. It's the same thing with a siloxane based grout sealer. You can seal your grout or brick, but in future if you want to add more sealer over top of what you have to provide better protection, the new siloxane sealer won't stick to the old.
You're much better off going with an ACRYLIC film forming sealer that doesn't suffer from this problem. Basically, a new coat of acrylic grout sealer will stick as well to old acrylic sealer as new latex paint will stick to old latex paint, which is pretty good.
PS: you don't need to know the rest.
Silicon rubber was first discovered when scientists tried to make the same plastics using silicon that they had made with carbon. Both silicon and carbon form 4 covalent bonds, and so it seemed plausible that silicon based plastics could be made exactly the way carbon based plastics were being made at the time. One of the first of these silicone plastics to be discovered was silicone rubber. At the time, about all that was known about it was that to make the plastic, you needed one oxygen atom for each silicon atom, and scientists had reason to believe the basic structure of the silicon based rubber was as follows:
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Si = O
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Si = O
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Si = O
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Now, KETONES are a class of hydrocarbon solvents with the general formula:
A
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C = O
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B
Where A and B can be any hydrocarbon group. If both A and B are methyl groups (-CH3) then you have "dimethyl ketone" or acetone for short.
If A is a methyl (-CH3) group and B is an ethyl (-CH2-CH3) group, then you have "methyl ethyl ketone" or MEK for short.
So, to name the new rubber made from silicon, scientists took the letters SILIC from the word "silicon" and the suffix "ONE" from the word Ketone, and put them together to form "SILICONE".
So, quartz and computer chips are made of silicon, but the caulk you put around a bathtub is siliconE; that "e" at the end coming from the word "ketone".
Well, it turns out that scientists were wrong. It turns out that silicone rubber doesn't have a ketone like structure at all. It in fact has a structure like this:
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Si - 2R
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O
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Si - 2R
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O
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Si - 2R
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where R can be anything, like a Hydrogen atom or a methyl (-CH3) group or and ethyl (-CH2-CH3) group, etc.
And that basic building block of
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O
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Si - 2R
is called a "siloxane" group.
If it says on the label "dimethyl siloxane" then each R is a methyl group. Similarily, "diethyl siloxane" would have two ethyl groups attached to each silicon atom in the chain.
Still, the name "silicone" rubber stuck, and so now all of our "silicone" rubbers are made out of siloxane groups of various kinds.
