Just to add to this, Venus's clouds already give it a .65 albedo...meaning that already it reflects 65% of the solar radiation back out into space. A shield would, at best, add the extra 35%.
Does it reflect or trap heat? Essentially if this is the case, then slowly removing the cloud structure around the planet, or at least making it less toxic and heat absorbent becomes the goal. Again, this was just one of many proposals that I've seen. It is clear that the planet would have to be considerably worked on before it became habitable.