Just got back from a month in Thailand. I had two weeks of work and then two weeks to travel. My wife came over for the second two weeks and after a couple days exploring Bangkok (Grand Palace, Wat Pho, Weekend Market, et al), we headed south to West Railay, arguably the most famous climbing locale in Thailand (and perhaps SE Asia). We flew from Bangkok to Krabi on Air Asia (cheaper than Thai Airways) and then took a taxi from the Krabi Airport to the beach at Ao Nang (400 Baht OW for the two of us). A 15 minute boat ride got us from the Ao Nang beach to the West Railay beach.
We stayed at the Sand & Sea Resort in West Railay, which was great (see Trip Advisor and other travel websites for reviews). The climbing in the Railay area is on limestone and there's plenty of it. The Railay area is a very popular beach destination and consequently there are lots of tourists. The locals have done a good job of promoting climbing as a half day or full day adventure activity, hence there are beginner rock climbing classes at the crags constantly. What this means is that the easier (5.7-5.10a) climbs are frequently busy with groups (at least during the Xmas season, which is when we were there).
We climbed at the 1,2,3 Wall, Muay Thai Wall, Escher Wall and Thaiwand Wall. All had good sport routes and were in very easy walking distance of the Sand and Sea Resort. A 60 meter rope and a dozen quickdraws will suffice for these areas. We also navigated through the cave that runs from the Escher Wall on Phra Nang Beach through to the Thaiwand Wall on the West Railay Beach. This is a very fun little outing. You definitely need a headlamp to get through the cave, which is perhaps 300 yards long and involves climbing bamboo ladders.
If you are interested in very overhanging sport routes and/or the young hipster scene (we are old and weak so neither of these options appealed to us), then Tonsai Beach is the place to go. It's about a 15 minute walk from West Railay.
After West Railay, we took a ferry south to the island of Koh Phi Phi Don. This island is extremely popular with college partiers, but also has good climbing and excellent diving and snorkeling. We stayed at the Phi Phi Inn, which was OK (we would look for alternate accommodation if we were to go back). If you want 24/7 partying while on the island, stay at the Tonsai Village portion of town. If you want a more relaxed scene, stay closer to the pier. The optimum for quiet, would probably be to stay to the left (west) of the pier (closest to Tonsai Tower). The town is small enough that you can easily walk anywhere within a few minutes. We climbed at the Tonsai Tower, which looms over the harbor. The routes here were also on limestone, but of a different sort than Railay. The rock was more akin to climbing in the Canmore, Canada area than El Potrero Chico in Mexico or the south of France.
We then headed north to Chiang Mai. We had originally hoped to travel from Bangkok to Chiang Mai via the overnight train, but it was fully booked, so we flew. Our accommodation was at the Yindee Guesthouse inside the old walled city portion of Chiang Mai. We climbed at the Crazy Horse Buttress, which is roughly 20 miles outside Chiang Mai. The easiest way to accomplish this is to go to the climbing shop in town and book transport through them. This costs 250 Baht/person (the exchange rate when we were there was about 30 Baht/US dollar) and includes transport in the back of a pickup truck to and from town, clean drinking water and a good Thai lunch at the crag. Going to and from the crag via the climbing shop transport there is a quick stop at a roadside eatery where you can use the bathroom and buy food and drinks (this is especially important at the end of the day, because you can buy beer to drink on the way back to town!).
The locals have done an incredible job of developing the Crazy Horse Buttress area. It is well signed, there are contoured paths to each of the cliffs, the bases of most of the walls have been stabilized and there are shade structures. A 60 meter rope and a dozen quickdraws will suffice at this area. The limestone here comes in two basic flavors – the grey, featured rock similar to El Potrero Chico and a yellow/orange variety that is reminiscent of the Yellow Wall at the Gunks. We did a bunch of great routes at the Crazy Horse Buttress. The local guides that use this area seem very squared away and the area has been developed with guiding in mind. There are enough moderate routes for both guided and non-guided parties to stay busy without crowding each other (unlike Railay).
When we arrived in West Railay we purchased a sport climbing guidebook to Thailand from the Hot Rock climbing shop (800 Baht). This 2010 guide covered Railay, Phi Phi Don, crags in central Thailand and the Crazy Horse Buttress. The guide looked very slick, but proved not to be particularly useful. Despite being published in 2010, the guide had incorrect bolt counts for almost every route, inconsistent ratings and didn't come close to including all of the established routes (I can hear the pundits now – not much different than the guides I have produced!). The other guidebooks that we looked at (with the exception of the Northern Thailand guide for Crazy Horse Buttress) seemed equally inaccurate/dated. Most visiting climbers we spoke with seemed of the same opinion. The takeaway is that you should expect, at least in the near term, to ask others about difficulty/quality and judge routes yourself rather than relying solely on the currently available guidebooks. This guidebook situation should not be construed as a big problem – after all, it's a sport climbing area, so it's unlikely you'll get in much trouble if you venture onto an unknown route.
Another issue unique to the seaside climbing areas in Thailand concerns the viability of bolts. Many of the older (pre-2000) routes were established with stainless steel expansion bolts and hangers. Due to the nature of the area (salt air, humidity, limestone, etc), many of these bolts and hangers are now suspect and the routes considered unsafe to climb. A large number of routes were more recently established with stainless steel glue in "loop" style bolts. These too have become suspect. The current standard is Hilti titanium "loop" style bolts with red epoxy. This configuration reportedly costs about 550 Baht each, which is a lot of money for Thai climbers. Consequently, there are many routes included in the guidebooks that have not been retrofitted with new titanium anchors and are reportedly unsafe to climb. It's also common to find a cluster of three bolts next to each other on a route – an old expansion bolt, a stainless steel glue in, and a new titanium glue in. This sort of "generational stacking" of bolts seldom occurs on climbs in the US.
In summary, it was a great trip and Thailand is a wonderful destination.