Can you make a whole model transparent?

As M39a9am3R wrote, you can achieve this by using a loop. However, if your model has submodels, you'll encounter a problem with just a single loop. To solve that issue, you can create a recursive function - a function that can call itself inside itself.

-- param: 
--     Instance model,
--     String property,
--     Undefined value
function setProperty(model, property, value) -- *1, 
 for _, obj in pairs(model:GetChildren()) do
  pcall(function() -- *2
    if obj[property] then -- *3
     obj[property] = value -- *4
    end
  end)
  setProperty(obj, property, value) -- *5
 end
end

-- Let's run the function
setProperty(game.Workspace.myModel, "Transparency", 0.7)

--[[
--Explaining further--
*1: model::Instance is root model (target), property::String is the name of the property you wish to change, value::Undefined is the value you wish to set the property to

*2: Adding a pcall to make sure the script continues even if it errors. You can additionally add an error-message - check pcall documentation/API

*3: making sure we can actually set the property by checking it's validity.

*4: setting obj's [property] to value. Treating it like a table.
myModel.Name can be written as myModel["Name"], and this works for other properties as well. We take advantage of this!

5*: We'll run obj through our function, in case it has any children
]]

You would need a loop to go through all the parts of the model,

for _,v in pairs(game.Workspace.MODELNAMEHERE:GetChildren()) do --So GetChildren makes a table, v is the value of the item.
    if v:IsA("BasePart") then --If v is a brick or wedge part or something like that, then the code to come will run.
        v.Transparency = .5 --The brick is changed to .5 Transparency.
    end
end