Hello, i am currently working on a portal 2 map in the hammer, but i am having some problems when i apply a skybox to the level. It seems that all the brushes turn transparent,
Here is some examples with and without the skybox:

I know i have to open the vmt file and remove something, but i am not sure where that file is loccated, i just used one that was in the hammer (difficult to explain)

Is your level leaking?

if you use custom skybox (for example from gamebabnana) and you entered the name of the skybox well into the map properties (the name of the textures but without the last 2 character, only the common first part) then must edit all 6 .vmt, changing/replacing the first row to "UnlitGeneric", and with VTFedit (free program) must edit all 6 .vtf and check the 'Clamp T' and 'Clamp S' checkboxes, and save. After these, there shouldn't be any weird glitch. OF course must pack all 6+6 file to the .bsp when publishing.

DaMaGepy wrote:
if you use custom skybox (for example from gamebabnana) and you entered the name of the skybox well into the map properties (the name of the textures but without the last 2 character, only the common first part) then must edit all 6 .vmt, changing/replacing the first row to "UnlitGeneric", and with VTFedit (free program) must edit all 6 .vtf and check the 'Clamp T' and 'Clamp S' checkboxes, and save. After these, there shouldn't be any weird glitch. OF course must pack all 6+6 file to the .bsp when publishing.

Well, you see, i use one that i found on the hammer texture browser, and i do not know where those textures are located :/

I don't know if you did, but you can't just apply any skybox texture directly to any brush face. Instead you must use the toolsskybox texture on every brush face where you want the sky to show up; then into Hammer go to Map > Map Properties and put the name of the skybox you'd like to use in the "Skybox Texture Name" property.