Weedkiller to prevent new weeds growing…

I’ve spent weeks strimming and weeding a big area that will eventually be covered with weed barrier/stones, but until that day I just want the area kept clear of weeds/grass. After clearing it, I sprayed with Roundup, but when new weeds/grass started growing, I found out that Roundup only kills active plants. I read somewhere that Pathclear prevents germination for up to 3 months, so a week ago I resprayed the area with Pathclear, but weeds/grass haven’t died off yet. Am I using the wrong weedkiller? I measured the water correctly so had the right mix.