Regarding Lascaux dry time, it's not as fast as Print Shield. It apparently uses less volatile solvents. I think that may be why it penetrates so much better, while Print Shield appears to clog the pores. There are reports of Print Shield turning into solid little balls before it hits the paper in hot dry areas.
I would let Lascaux dry for a couple of hours. I don't think there is a risk of trapped solvent vapors in the long run because they will go through the back of the matte paper. In fact, water vapor and oxygen slowly go through these coatings.
I'll be testing a few approaches with and without Lascaux and see if I can tell any differences.
Paul
www.PaulRoark.com
From a physical longevity perspective, I'd still be very hesitant to use Lascaux, or any other solvent-based acrylic spray, as the first coat. Since this is the coat that soaks into the paper base and holds the paper fibres and image layers together in one big, hardened mass of acrylic, it needs to be flexible - if it's brittle, it could just develop micro-cracks over time as the paper flexes and bends, eventually breaking down enough that the Lascaux-image-inkjet coating-paper layer crumbles away and fails entirely. You can see a similar kind of failure occuring in some oil paintings, with the brittle, inflexible layer of oil paints crumbling and flaking off the canvas base as it expands and contracts.
I still like the enhanced light/gas fastness that Lascaux seems to provide, though - I wonder if it would provide the same degree of protection when applied as the second layer, so that it is in front of the image, rather than encasing it in a brittle layer, and is itself supported on both sides by flexible water-based acrylic. I guess this depends on the mechanism of protection provided by the Lascaux spray - whether it is a chemical property of the spray itself (unlikely, since the acrylics it contains are the same basic family of acrylics as those in water-based coatings), or whether it is merely a result of it sinking into the paper and completely sealing the pigments from the atmosphere in all directions (unlike a typical coating, which seals it only from the front). If it is the latter, then I would guess a water-based coating would do just as well and produce a more flexible coating, provided the coating and paper combination are such that the coating can sink in properly (apparently watered-down Timeless will sink into Breathing Colour matte papers and Epson Watercolour, but not into Canson Rag Photographique).
I'd also be inclined to spray the back of the print with a water-based acrylic, both as an anti-curl layer and to seal the back of the print against moisture and pollutants. But the print would have to be completely dry and out-gassed in order to do this, and the water-based acrylic would also have to dry via evaporation rather than by absorption of the solvent (water) into the paper base, in order to avoid trapping the water in the spray within the paper.
In any case, spraying gloss coatings on matte paper seems to be the most promising way to produce a baryta-like gloss finish (or any other kind of finish, from satin to ultra-gloss) without the inherent fragility of baryta or the uncertain durability of a coating that isn't permanently bound to the paper substrate.