vellum and spray for toner darkening for PCB production
I'm not aware of your spray and how it works, but there are special toner density sprays, which you often get from shops selling everything for PCB etching. This sprays contain a solvent which dissolves the toner on the paper a just a little, so it fills the small gaps and pores of the paper. It evaporates completely, and there is no concern about any residues left on the paper. But I guess your spray is not that different. 

I had a Brother HL2030 laser printer, and its printouts got really really dark with this spray. 
Unfortunately, the printer stopped working, and I have a Brother HL2340 now. And the spray has absolutely no effect on its printouts. The toner cartridge is almost empty, and I've already ordered a new, 3rd party one. Maybe, the spray will work with that. 

So, if such a spray works depends on the type of toner, too!

To my experience, the darkness of the toner heavily depends on the paper. On transparencies, it's far from being really dark, and even on translucent paper, the result is not so good.
In contrast, the printout is really dark on normal paper, and to my experience, the density spray has the best effect on this paper, too.

Now, there is a second kind of spray, which makes normal paper translucent, as oil does. I only know Transparent 21 from Kontakt Chemie, a german company specialized on sprays for electronics. They sell at least europe wide, not sure if also world wide.

I didn't try, but a few drops of oil should do the job, too. Just clean the PCB thoroughly but gently with some dish liquid or so. But oil could also dissolve the toner. Glycerine should not, and it's easy to rinse it off the PCB, but it might swell the paper. Just try out.