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I've got a section of hardwall plaster (on top of brick) where it looks like the plaster has 'swelled' and puffed out. In our garage, the previous owners left bags of hardwall plaster, so I'm thinking one way to fix this is to remove all of the swollen/cracked plaster and mix up new hardwall plaster and apply.
Is this the best approach and do you have any tips for applying it to brick and getting a smooth finish?
You’re on the right track with removing the damaged plaster @Simon_B, but there are a few things to keep in mind. If the plaster has already puffed out and cracked, it’s a strong sign that the surrounding plaster may also be poorly adhered, even if it looks intact. Start by carefully chipping away all the areas that are loose or proud of the wall, then check the plaster around that section by gently tapping or pulling it. Any areas that come away easily should be removed as well. Be prepared that you may need to redo a larger patch than just the visibly damaged section.
Once you’ve cleared the loose plaster and have a solid surface, you can apply your new hardwall plaster. First, brush off dust and debris from the brick, then wet the brick slightly to help adhesion. Apply the plaster in thin, even layers, pushing firmly into the brick to get a good bond. You can use a straight edge to level the plaster as you go, and then a trowel to smooth the surface. After it starts to set but is still workable, lightly float the surface to remove trowel marks and get a consistent finish. For the best result, finish with a clean, damp sponge or trowel to produce that smooth hardwall look.
If you do not have prior experience with hardwall plaster and if this turn out to be more than just patching that area, you should consider removing all the plaster and installing plasterboard instead.
A final note on safety: There’s a good chance it actually is hardwall, but unless you are 100% sure, and especially if your home was built before the mid-1990s, there is a possibility that asbestos could be present in the wall. You definitely do not want to disturb asbestos-containing material. If the property was built after the mid-1990s, that would rule it out, but until verified otherwise, assume there could be asbestos and take appropriate precautions.
Here's a helpful guide: How to hard plaster a brick wall.
Please let me know if you have any questions.
Mitchell
Thanks Mitchell! I'll be sure to follow that advice and will reach out if I run into any issues 
