I need a glue that won't turn opaque or run

Question

I'm trying to glue something to a rock and need a glue that won't turn cloudy over time. I've spoken to several glue manufacturers to no avail. I have also tried about 6 brands so far and none remain clear even if they were clear before drying. The closest I've gotten is with Elmer's. It dried clear but within 6 months became cloudy. Any suggestions? Thanks for thinking about this. I should also mention I need the glue to stay in a "blob" in certain areas so the paint on the underlying rock surface will show through the well-defined dollop of glue. As I said, I used Elmer's which turned cloudy but it didn't run, so that was half the battle. Sorry I didn't mention this in the first version of my question. I did get some answers one being heat-activated adhesive which I have not tried as yet. I have tried Gorilla Glue (dried cloudy) and Duco Cement (was very runny).

Answer 1

Use a clear liquid epoxy resin, construction adhesive, 'Gorilla Glue', or 'Iron Grip' silicone adhesive:

• Liquid epoxy resins dry clear and hard, bonding stones that may not fit snugly together. It may make some stones look wet in the area of application. Since this resin is a liquid, it runs a bit after you apply it, so it can pool beneath the stones if you use too much. Epoxy resin works well on stones, both large and small.

  
  ^{source: smaller scale than what the OP might have in mind, but still a valid example of an end result. Be sure to work safer than the author, though.}

  As per this site, be sure to warm up the resin before application (though not with a heat gun or similar device), as this may help prevent it from turning cloudy. In case of outdoor use, be sure to pick an epoxy that is UV stabilized.
  Additionally, make sure you apply it in a dry environment and protect it from getting wet (also while curing), and don't overmix it. ^*

  Update: you won't be able to get blobs with epoxy resin, unfortunately.

The following glues are commonly or regularly used for glueing brick walls together, or glueing stones to a vertical surface.

• Construction adhesive, offered in a caulk-style tube, creates bonds strong enough to hold larger stones together, such as baseball-style stones or river rocks around the perimeter of a homemade garden pond.

  Make sure you're using a clear variant.

  ^source

• 'Clear Gorilla Glue' (no affiliation):

  The strength and versatility you have come to expect from Gorilla Glue is now available in an easy to use, crystal clear, non-foaming formula. Clear Gorilla Glue is incredibly strong and is ideal for indoor and outdoor applications and bonds virtually everything.

  See here for an example of mortar replacement. Mind that the regular unclear version is used.

• 'Iron Grip Adhesive Silicone' (no affiliation):

  Made from 100% silicone, GE Iron Grip adhesive works fast on almost any porous and non-porous surface, including aluminum, steel, PVC, glass, concrete, fiberglass, polystyrene, plywood, drywall, ceramic, porcelain, stone, laminate, wood and MDF – indoors and out – all without any extra surface prep.

  Clear curing. Here's a commercial video, showing the glue used on concrete blocks. As always, check the reviews.

^{* More useful information on the application of epoxies can be found here.}

Answer 2

Liquid Nails Clear is a Selleys product made in Australia. We use liquid nails here as an all round building and construction glue, including masonry, metal, timber.

Their technical support people tell me:

“Liquid Nails Clear dries very clear with a clear bond line so you have a mess free bond line (you shouldn't be able to see it unless you look at it with an eagle eye.)”

Their instructions say it resists water, and. "24-48hrs strong bond. Cures in 14 days."

UPDATE:

A test using two bricks

Given the lack of actual photos (even from Selley's) I have done my own test using two house bricks. As you can see the join uses much more glue than necessary but wanted to test and then show its transparency. I think it passed. This product gets tacky very quickly. It's a glue I would always wear latex or other gloves with. (I do not need more chemicals in my body.)

Photo 1: Two bricks glued together using Selley’s Liquid Nails Clear:

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Photo 2: One strong joint. "24-48hrs strong bond. Cures in 14 days." (Bricks are not resting on ground by the way).