Do you have any tips for me? I have a very old property and will be resealing the windows and doors to the brick work and want to try and use burnt sand and linseed oil.
I'm sorry I have taken so long to reply, a combination of having been away from my family since before Easter and having my mind on other things.
The first suggestion is to make a small hawk out of a scrap of thin ply about 6" square. I used a piece of dowel or broom handle as a handle. Two perpendicular upper edges of the hawk need a raised lip out of scraps of timber about 1/2" high to push against with what Marshalltown refer to as a tuck trowel.
The sand and oil need to be mixed so that it has a damp sand consistency, use as little oil as possible at first then add a little more. If there is too little oil it dries dusty/crumbly but too much oil is also not good.
Decide the width of the fillet and choose your trowel width accordingly, you can use plasterer fine tools to help refine the shape but it is quite a delicate operation.
Both the brick/stone where the mastic will be laid need to be carefully primed with just the oil first quite lightly or it will spread, using masking tape can help with applying the mastic in a straight line but the oil application needs a gentle touch or it will spread and stain.
The mastic is placed on the trowel and it is picked up on the trowel by pushing it against the raised edge, it should stick to the trowel but again this is a very delicate operation and it will easily fall off. You then transfer it to the area between wood and stone as a fillet and press it home.
I cant over emphasize how delicate this needs to be, it really does take some practice, and getting the balance between sand and oil right will also take some practice.
I am by no means a professional but I have done a few windows over the years and was shown by an old-school master craftsman.
I hope this helps a little, do ask if you have any questions, I will try to help. I haven't used Mason's mortar for some time but they were always incredibly helpful.