Because of how a water slide decal works, applying one to an unsealed surface isn't really a suitable job for a decal. Without using specialist setting and softening solutions, you're going to trap air bubbles beneath the film, which leads to the unsightly phenomenon known as "silvering". When you see it, you'll know it, and understand why it's called silvering.
If you don't want to buy specialist solutions, your most likely way to successfully do this is to apply your decal to a very thin wet coat of Future, and let that draw the film into the surface as it cures. It's quite magical stuff. For something as open pored as wenge I would apply Future very tightly to the decal outline, apply the decal as I described, and then either gently brush over another thin coat of Future or, use a softening solution applied very carefully throughout the decal drying time.
A softening solution can be as simple as a 1:1 mix of white vinegar and distilled water, which you apply to the surface and then brush VERY lightly over the decal after it has been applied.
For really tricky work, use either Microscale Micro Sol or GSI Creos Mister Mark Softer. They work by literally melting the decal into the surface. The Microscale is much more user friendly than the very aggressive GSI solution. I would strongly recommend trying them on a test piece first to get an idea of how they work.
