FENDER Patented Callus Builder & Finger Strengthener – Resistance Finger Exerciser for Guitar, Bass & Violin – Guitar Finger Trainer & Grip Strength Tool – Ideal Guitar & Bass Accessories (Tensions: RED=8-Lbs; BLACK=15-Lbs)

It has a magnetic on the back, it's also a potato chip bag clip ( too funny but true) It has strong resistance and starts to build calluses on fingers after just a few days!

Easy to use! Already feeling the finger tips getting the work out! After two days Great product 👌

I’m a professional orchestral musician and I bought this to keep my calluses up when I’m away from my instrument. After a couple of weeks of using this my calluses were bigger than ever and my hand strength is also up. The black strengthener is probably more intense than a guitarist would ever need, but this device perfect for working professionals who play everyday.

Seems to help maintain callus as described and is comfortable to hold. Useful to fidget with while I'm working on other things. Makes a slight creak as the plastic hinge wears. And the magnet on the back can't even hold it's own weight on a metal surface so that part is useless. Has not broken yet but curious how long it will last. For 20 bucks I expected a higher build quality.

I recently started up playing guitar again after taking a hiatus for a couple years. Unfortunately, the calluses I built up long ago faded away. I bought this set of callus builders to see if I could accelerate the process of building up callus. I debated buying only the 15 lb. device but opted to pay a little more and buy the set of both the 8 lb. and 15 lb. devices. I’m glad I did. Both weights will strengthen your fingers and help build callus, but my 70 year old fingers found the lighter one easier to use, at least until I can build up more finger strength. But both are definitely helpful, and they’re easy to use while watching TV. My only complaint is these devices are somewhat small. I have to bunch my fingers up a little bit to fit all 4 fingertips on the device to build callus. It would have been better if these were about 1/4 inch wider. Overall, I’m pleased with these callus builders. They’re kind of expensive for glorified potato chip bag clips, but they work as advertised, and I’m sure I’ll get a lot of use out of them.

I work in IT. My hands are soft. I type on keyboards all day. But I want to play guitar like Stevie Ray Vaughan. The problem is, my fingertips are weak and sensitive. I bought this little gadget to keep at my desk. It has buttons that simulate guitar strings. I sit in Zoom meetings, nodding along to the boss talking about "synergy," while under the desk, I am tirelessly clicking away. Click. Click. Click. The Result: My fingertips are now hard as rocks. I can play for hours without pain. My coworkers think I have a nervous tic, but that’s a small price to pay for rock and roll

This is my 2ed one and booth springs broke in about 3 months . Im not buying another one. Will ask for new spring.

I am not sure if this works as I was expecting it to. Quality is ok, plastic and all. Ease of use. But how it help for getting blister for playing your string on the guitar. Was a waste of my money.