Yeah. They're weird little bastards. I mean they obviously like to feed on old leaves. I usually find the largest structural damage there (silvery/bronze scraping scares in huge irregular blobs on the underside of leaves). However, once you have an infestation they seem to target the new shoots like it's the most delicious buffet. So what ends up happening is that all new growth comes out all weird and damaged while the old leaves slowly succumbs, it can quickly spiral into a critical condition for your plant since it can't really save itself.
As I said though, insecticidal soap is your best friend. Just use true soap and not dishwashing liquid or something else strongly detergent. Check my posting history on some information on this. E.g.
here and the post I refer. 1 % is enough as long as you make sure to actually wet everything thoroughly and let them dry slowly, preferably somewhere where you have 90% humidity. High enough humidity and you could probably get away with 0.25 % if your water isn't that hard. But do at least 1 % since phytotoxicity should be practically non-existent as long as you use a long-chained pure-fatty-acid SOAP (I've even wet plants 2 cm out of the grounds with no problems). Don't take any chances.
If the leaves start drying before you've moved them, re-spray them. I can't emphasize the "let 'em dry slowly"-recommendation enough; Soap truly only kills via suffocation (hugely increased wetness of the water in combination with their passive breathing tubes) but thrips can survive "holding their breath" for a little while. Try to start the spray session of each plant by high pressure to wash them off as well and then turn to a fine mist to cover everything. Then you increase your chances of removing the largest amount of the reproductive population possible. Oh, this probable goes without saying but COVER EVERYTHING. On top, under leaves, inside flowers, between buds and stems, along the woody stem, lightly mist the top soil, wash off the pot, etc. Everything. Don't let them camp out and re-colonize the next day.
No, sand won't do shit I'm afraid. Screw the fungus gnats. They seldom do any real damage (unless your soil is always soggy). Just water more appropriately and put up some yellow sticky paper to catch 'em. Thrips is a real threat on the other hand. Not immediately but if you neglect the plants they will become quite stunned, might get viruses and could even die. Focus on the thrips.
But don't worry. You can get this under control. You just need to be hypervigilant and avoid pro-crastinating "spray days" for a while. May your thrips not rest in any peace whatsoever.
