I have bubbles appearing right down to the original wood after the new stain is put on. What wood cause this?
1- Moisture in the wood before applying your solid stain.
2- A chemical reaction with the old stain.
Both of these will cause bubbling.
Poor preparation of the substrate before applying the new finish is a likely cause. If the old finish isn't well adhered to the wood, it will absorb moisture from the new stain as it's drying and will swell and bubble up. That's why it's important to pressure-wash and scrape the siding (or wire-brush rough surfaces) before repainting to remove any loose finish.
The bubbling I'm describing would appear within a day or two of applying the new finish.
If bubbling appears long after refinishing, the most likely cause is moisture getting into the underlying wood through cracks in the stain, at siding seams, or through wicking from points where wood comes in contact with the ground. (It's almost always moisture that causes finish failure.)