I'm at the stage of raising the walls and I'm already seeing moisture stains on some walls even though it barely rains. My compadre says I should apply a waterproofing agent before I finish, but I'm not sure whether to do it now or after everything dries completely.
Don't apply anything while the walls are still releasing moisture, because the waterproofing won't adhere properly and it's going to peel off later. What few people mention is that before sealing, you need to identify where that moisture is coming from: if it's capillarity from the foundation, you need a moisture barrier at the base, not just waterproofing on the wall. Wait for the walls to dry completely (it can take weeks depending on the climate and material), then clean the surface well and only then apply the waterproofing. If you don't solve the source moisture problem first, the waterproofing is only going to cover up the mess for a while.
Do those stains show up at the base of the walls or spread out all over? What your buddy's telling you makes sense, but timing is everything: if you apply the waterproofing now on wet walls, it won't stick properly and you're just throwing money away. Best thing is to let the mortar cure completely (usually two or three weeks depending on the weather), then seal the joints real well and only then apply the waterproofing. If you're seeing moisture at this stage, it's probably from runoff or the ground being waterlogged, so also make sure the perimeter drainage is working right before you start messing with waterproofing products.
I had a real mess with this on a job a while back: I applied the waterproofing when the walls were still damp and it peeled off after a few months. The moisture you're seeing now is normal during construction, but you've got to let it dry completely before applying anything, otherwise the product won't stick.
The ideal thing is to wait for the walls to cure properly (depends on the climate, but we're talking weeks), and then apply the waterproofing. In the meantime, make sure there's good ventilation on site and that rainwater doesn't pool at the base; that's what does the most damage.
What you're missing is figuring out where that moisture is coming from: if it's rising damp from the ground, surface waterproofing won't solve anything for you. Before you throw products at it, verify that the base is properly isolated from the soil (waterproofed base, polyethylene barrier, whatever you put down). Once you're confident about that, wait for the walls to actually dry out - it can take weeks depending on thickness and climate - because like they said, applying over wet is just throwing money away.
The key is waiting for the walls to dry completely before waterproofing, but here's what nobody tells you: while you're waiting, you need to improve ventilation and drainage around the construction site, because if the moisture is coming from below (capillary action), no surface sealant is going to save you. Those stains you're seeing now are normal during construction, but if after two weeks of good weather they keep showing up at the base, that's a sign there's a drainage problem you need to fix before applying any product. Your buddy's right about the waterproofing, but apply it once everything's dry; putting it on wet walls is just throwing money away.
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